Nitrogen Spray Refrigeration System For Perishables

Abstract

A system for more efficient refrigeration of perishable products in trucks and railcars is provided by spraying cold fluid into the product chamber at effective flow rate resulting from maximum flow rating between 30 and 110 lbs. per hour equivalent pure nitrogen per 10 foot average length of storage chambers having cross-sectional area between 40 and 110 ft.sup.2, and through spaced small openings in a spray conduit of between 0.002 and 0.009 inch.sup.2 per 10 foot average length.

Parent Case Text

BACKGROUND OF THE INVENTION

This is a divisional application of Ser. No. 776,331 filed Nov. 18, 1968 by W. R. Jehle, now abandoned.

Description

This invention relates to a method of and apparatus for highly efficient nitrogen-rich liquefied gas intransit refrigeration of perishable product wherein the refrigerant is sprayed into the product chamber.

The intransit refrigeration of perishables by spraying cold cryogenic liquid from a liquefied gas storage body into the perishable product storage chamber is widely practiced as described in Kane et al. U.S. Pat. No. 3,287,925. The conventional practice has been to use sufficiently high maximum flow ratings of refrigerant for rapid cooldown of the product storage chamber following initial product loading or doop openings, i.e. greater than 200 lbs. per hour liquid nitrogen per 10 foot average length of chamber. In this manner the cryogenic liquid spray system could minimize the period a perishable product, e.g. lettuce, is exposed to high ambient temperatures at which deterioration is accelerated. This was recognized as a significant advantage over mechanical refrigeration systems where the cooldown rate is severely limited by the need to mechanically circulate environment gas over cooling coils and thereafter refrigerate the product with the cooled gas.

Certain problems have developed in connection with the prior art liquid nitrogen spray refrigeration system, including relatively high liquid nitrogen consumption rates. This necessitated refilling of the liquid nitrogen storage container at more frequent intervals than was convenient or economically desirable. Another problem was that the relatively high refrigerant flow rates made it more difficult to maintain substantially uniform temperatures through the entire length of the product chamber. The flow of refrigerant is normally controlled in response to temperature sensing means, e.g. a thermostat, positioned within the gas space of the product storage chamber so that the refrigerant "on" cycles required to reach the thermostat set point temperature were relatively short. During these periods the chamber sections and product closest to the openings in the overhead spray conduit tended to be overcooled while the remote sections were undercooled.

It is an object of this invention to provide an improved method of and apparatus for cold nitrogen-rich spray refrigeration of perishables.

Another object is to provide such method and apparatus in which the nitrogen-rich liquid consumption rate is lower than heretofore realized.

Still another object is to provide such method and apparatus in which more uniform refrigeration temperatures are realized throughout all sections of the product chamber.

Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.

SUMMARY

In the method aspect of this invention, pressurized nitrogen-rich liquefied gas is provided in a thermally insulated storage container associated with a product storage chamber having thermal insulation sufficient to limit the heat inleak rate to below 0.10 Btu/hr.-.degree.F.-ft.sup.2 inside surface area.

The gas temperature within the storage chamber is monitored, as for example by a temperature sensing bulb or thermocouple, and cold fluid is dispensed from the storage container in response to the monitored gas temperature. This cold fluid (either liquid or gas) is sprayed as a multiplicity of discrete streams spaced from each other along the length of the storage chamber for refrigerating the perishable product therein. The effective rate of dispensing is that provided by a spray header having maximum continuous flow rating of between 30 and 110 lbs. per hour equivalent pure nitrogen per 10 feet average length of chamber for chamber cross-sectional area between 40 and 100 ft.sup.2 so as to maintain the monitored gas temperature in a selected temperature range of -10.degree. to 60.degree.F. This flow rate is of course far below the aforementioned 200 lbs. per hour nitrogen per 10 foot average chamber length maximum flow rating characteristic of present intransit spray nitrogen refrigeration systems. Tests have indicated that under some operating conditions this invention permits about 30 percent reduction in nitrogen refrigerant consumption over sustained periods with no noticeable loss of product refrigeration effectiveness.

In the apparatus of this invention, the aforedescribed liquid nitrogen storage container and product chamber are provided along with spray conduit means positioned within the upper portion of the storage chamber and extending substantially the entire length thereof. Openings of equivalent diameter less than 0.07 inch are spaced along the spray conduit length for discharging a multiplicity of discrete cold fluid streams into the storage chamber. The openings have a total area of between 0.002 and 0.009 inch.sup.2 per 10 foot average length of chamber whereas the prior art employed substantially higher opening total areas. At areas below 0.002 inch/10 ft. average length, insufficient nitrogen refrigerant flow would be provided into the storage chamber to recover from heat inleak due to door openings within a reasonable period of time.

In this apparatus, fluid discharge conduit means communicate at one end with the liquid nitrogen container and at the other end with the overhead spray conduit. Fluid control means are provided comprising a temperature sensing element positioned within the storage chamber and a control valve operably interposed in the fluid discharge conduit means. The control valve is connected to the temperature sensing element to be responsive to the storage chamber temperature.

The term "average length" of chamber is used herein because a particular 10 foot section of chamber length may not have the nitrogen-rich flow rate or spray opening total area required to practice the method and apparatus of this invention. The aforementioned ranges are based on the total nitrogen-rich flow rate or spray opening area divided by one-tenth of the chamber length used to store product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view taken in cross-sectional elevation of a truck or semi-trailer incorporating one embodiment of the invention.

FIG. 2 is a schematic view taken in cross-sectional elevation of a railroad car incorporating an alternative embodiment having gas circulating means.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 illustrates an embodiment in which a mobile thermally insulated storage chamber 11 is provided for holding perishable product 12. This chamber 11 may be of standard construction for typical mobile refrigerated chambers, e.g. reinforced aluminum siding outer walls, plywood panelled inner walls and asbestos or plastic foam insulating material between the two walls. The thermal insulation 13 must be sufficiently effective to limit the atmospheric heat inleak rate below 0.10 Btu/hr.-.degree.F.-ft.sup.2 inside surface area, typical materials providing overall heat conduction rates of about 0.05-0.07 Btu/hr.-.degree.F.-ft.sup.2. A higher quality thermal insulation is not economically justified because the chamber is not airtight, access means such as rear doors being required for insertion and removal of the refrigerated perishable product.

A thermally insulated container 14 is associated with product chamber 11 for storing pressurized liquid nitrogen or nitrogen-rich liquid such as air. The term "equivalent pure nitrogen" as used herein refers to refrigeration content, as compared to liquid nitrogen. Because the thermodynamic properties of liquid air and nitrogen are so similar, they are considered equivalent for purposes of this invention.

The construction of container 14 is well-known and usually includes an outer shell completely surrounding an innerstorage vessel to form an evacuable insulation space therebetween, as for example depicted in Loveday et al U.S. Pat. No. 2,951,348. This space is preferably filled with an efficient solid insulating material, as for example alternate layers of radiation-impervious barrier such as aluminum foil separated by low conductive fibrous sheeting, as for example glass fibers described in U.S. Pat. No. 3,007,596 to L. C. Matsch. To remove gas accumulating in the evacuated insulating space, an adsorbent material such as calcium zeolite A or a gettering material such as powdered barium may be provided therein to retain a high level of insulating quality.

The vessel within storage container 14 is filled with liquid nitrogen by means well known to the prior art, such as for example connecting a source of liquid nitrogen stored at above atmospheric pressure to the container. If the liquid nitrogen is stored at a pressure below the operating pressure of container 14, a suitable pump would be employed and usually additional heat would be added to the pressurized liquid before transferring it into container 14. The liquid nitrogen is preferably charged into container 14 and stored therein at saturated conditions and at temperatures corresponding to a vapor pressure above 10 psig. with the entire liquid and vapor substantially in equilibrium. If the aforementioned highly efficient insulation is used, there is no appreciable amount of heat inleak to the inner storage vessel of container 14 and the stored liquid nitrogen is dispensed only by this as-charged vapor pressure. Alternatively the liquid nitrogen may be charged to container 14 under non-saturated conditions and even in the subcooled state. Under these circumstances it would probably be necessary to provide means for building sufficient internal pressure on demand to discharge the liquid. Those skilled in the art will appreciate that this heat may be introduced externally, using the wellknown pressure building coil. The latter includes a liquid discharge conduit, an atmospheric heat vaporizer and a conduit for returning the resulting vapor to the container gas space (not illustrated). As still another variation known to the art, a less efficient heat insulating material may be used so that sufficient atmospheric heat inleak is available to vaporize sufficient stored liquid refrigerant to form gas pressure to insure liquid discharge on demand.

It is preferred to store the liquid nitrogen refrigerant at pressure below about 100 psig., because at higher pressures the spray conduit openings would be impractically small, and also inherent lag characteristics of presently known temperature sensing elements would make adequate control of the liquid refrigerant withdrawal more difficult. The storage pressure is preferably above about 10 psig. to provide sufficient driving force for substantially uniform distribution of cold fluid through the spray orifices. Storage container internal pressures of 15-22 psig. are commonly used.

Liquid discharge conduit 15 is joined at one end to storage container 14 and has control valve 16 therein as part of a liquefied gas flow control system. The latter includes temperature sensing element 17, as for example a bulb positioned within the storage chamber 11 gas space. This bulb is connected by signal transmitting means 18 to temperature controller 19, and signal transmitting means 20 provides communication between the controller and control valve 16 in liquid discharge conduit 15. The flow control means may be electrically or pneumatically operated.

The other end of liquid discharge conduit 15 is joined to overhead spray conduit 21 having a series of spaced openings 22 therein. Spray conduit 21 preferably extends substantially the end-to-end length of product storage chamber 11 for discharge of cold fluid into such chamber as a multiplicity of discrete streams. Openings 22 may be oriented either horizontally or slightly downwardly in the conduit's circumference, and are distributed along the conduit to provide total area of between 0.002 and 0.009 inch.sup.2 per 10 foot average length of chamber. Openings 22 may be of any shape, although a circular configuration is preferred for uniform symmetrical discharge of sprayed refrigerant. The openings should have an equivalent diameter of less than 0.07 inch, i.e. an area smaller than the area of a circle of this diameter. Holes of 0.07 inch equivalent diameter or larger would be so few in number and so widely separated that the individual fluid spray streams would be isolated. With such large holes the refrigerant could not be introduced in a uniform pattern from one end to the other end of the chamber over the product and thus provide uniform cooling and temperature distribution.

In a preferred embodiment the total area of the spray openings is between 0.004 and 0.008 inch.sup.2 per 10 foot average length of the product chamber.

It should be understood that the flow rating of 30-110 lbs. per hour equivalent pure nitrogen per 10 foot average length of chamber, required by the method of this invention, is the maximum rate which might be employed with the control valve continuously open, and not the effective flow rate experienced while the control valve is open during normal cycling operation. As used herein, the term "effective flow-rate" is defined as the total refrigerant flow divided by the control "valve open" time. This includes the refrigerant used to initially cool the product to the desired low temperature level and the refrigerant consumed to recool the product to this level following warmup due to periodic opening of the chamber doors. The reason why the maximum flow rating criteria is used is that it eliminates variations such as cycle "on" time due to the ambient temperature. The effective flow rate during cycling operation is affected by the ambient temperature, i.e. higher for relatively high ambient temperature and lower for relatively low ambient. Tests have shown that for a particular spray header and liquid nitrogen storage container discharge pressure, the refrigerant flow rate increases with "on" time up to a maximum continuous rate. When the control valve is first opened and the spray header is warm, it initially discharges virtually 100 percent gas. As the spray header cools down with time, an increasing percentage of liquid is discharged through the openings. After continuous operation of 30-90 minutes (depending upon flow rate and heat capacity of the spray header) the system reaches substantially stable equilibrium condition and the spray header discharges virtually 100 percent liquid. The flow rate under such stable condition is the "maximum rating" as used herein. In a preferred embodiment of the present method, the aforementioned maximum flow rating is 40 to 90 lbs. per hour equivalent pure nitrogen per 10 foot average length of product storage chamber.

In the normal cyclic operation the temperature controller usually terminates refrigerant flow before the maximum rate is reached, and it has been found that the effective flow rate of an uninsulated spray header is on the order of 50-80 percent of its maximum continuous rating. Accordingly, the total area of the openings should be sufficient so that the desired refrigeration may be obtained at the effective flow rate.

In a preferred embodiment the spray conduit with spaced openings is provided in sections of standard length which can be joined end-to-end, depending on the length of product storage chamber. The latter may be relatively short, as for example the 8-10 foot truck body section of an ice cream local delivery truck, or very long, as for example a 40 foot semi-trailer or 50 foot long railway cars. The preferred sections for uninsulated spray conduits are composed of 1/4 inch IPS brass pipe 57 inches long with four 0.038 inch diameter circular holes spaced 11 to 15 inches apart. In particular, the four holes are located the following distances from one end: 10, 21, 36, and 51 inches. Whereas the heretofore employed nitrogen spray conduits had 1/16 inch diameter (0.0625 inch) circular holes, it has been discovered that smaller size openings discharge a more finely divided liquid spray which in turn affords more uniform refrigeration distribution beneath the openings. Tests have shown that with even very long spray conduits (40-45 feet), the pressure drop is negligible so that each 0.038-inch opening passes approximately equal quantity of cold nitrogen fluid during operation.

The aforedescribed spray conduit sections have been connected together for use according to this invention, in various size trucks (about 56 ft.sup.2 cross-sectional area) and railroad cars (about 95 ft.sup.2 cross-sectional area) as listed in Table A. ##SPC1##

It should be noted from the foregoing table that the total length of the standard sections for any particular product chamber is not necessarily equal to the length of the product chamber. In such instances, portions of blank pipe (without openings) are used to connect the standard sections of spray conduit. For example, a 2 foot long blank pipe may be placed between two standard 57 inch long spray sections for a 17 foot chamber, and a 4 foot long blank pipe may be positioned between two standard sections for a 20 foot long chamber.

In operation, liquid nitrogen is discharged from storage container 14 through conduit 15 to overhead spray conduit 21 and thence through openings 22 into chamber 11 for refrigerating perishable product 12. Refrigerant flow is responsive to the chamber temperature as sensed by element 17. The controller 19 is set to maintain the chamber temperature within a predetermined range, depending on the nature of the product. For frozen commodities, as for example ice cream, the temperature should be about 0.degree.-10.degree.F. whereas with freeze-sensitive commodities such as lettuce, the temperature should be 35.degree.-45.degree.F. It should be understood that this invention may be employed to refrigerate any type of temperature-sensitive product irrespective of whether the product is to be maintained in the frozen or unfrozen state.

The cold nitrogen fluid may be discharged through the spray openings 22 as a liquid, liquid-vapor mixture or entirely vapor, depending on the heat transfer characteristics of the particular system. For example, in the FIG. 1 embodiment only a relatively small portion of the liquid nitrogen discharged from container 14 is vaporized by heat from the environment gas upstream the openings. In other systems contemplated by the invention, a heat exchanger may be provided upstream of spray header 21 to vaporize part or all of the liquid nitrogen. Such a heat exchanger may be located within or outside the product chamber.

Another contemplated variation is a spray header with thermal insulation. Whereas non-insulated spray headers are satisfactory for systems refrigerating frozen products, it may be desirable to position thermal insulation around the spray header for systems refrigerating freeze-sensitive products, e.g. fruits and vegetables. This is because high convective heat transfer to the cold non-insulated spray header may cause freeze damage to the nearby freeze-sensitive products. Also, dripping water from ice frozen on the spray header during operation may damage product cartons and is annoying to unloading personnel.

Although the spray header may be located anywhere within the upper portion of the product storage chamber, it is preferably located along the upper portions of a side wall near the ceiling. In this location, all spray openings are placed along one side of the header and preferably directed approximately horizontally across the product, and if moisture forms it will drip down along the wall of the storage chamber.

In certain types of cold nitrogen-rich spray intransit refrigeration systems, usually those equipped with thermally insulated spray conduits, means are provided to circulate the product chamber environment gas for more uniform end-to-end temperature distribution. The need for gas circulation means, e.g. a fan, is greatest in long haul service (e.g. over 400 miles between loading and unloading points or requiring transit time greater than about 48 hours) and in particular in relatively long chambers containing products to be refrigerated at above freezing temperature to avoid deterioration. The fan may be electrically driven from a truck battery or generator, or powered by energy from the expansion of warmed pressurized nitrogen gas as described and claimed in copending application Ser. No. 643,709 filed June 5, 1967.

FIG. 2 illustrates a railroad car embodiment in which elements corresponding to those previously described and illustrated in FIG. 1 have been identified by the same numeral. Other elements which differ from FIG. 1 are described hereinafter, along with their functions.

Thermal insulation 21a is provided around overhead spray conduit 21 to avoid product freeze damage and moisture drippage. Second liquid discharge conduit 23 having control valve 24 communicates at one end with liquid nitrogen storage container 14 through first liquid discharge conduit 15. Alternatively one end on conduit 23 may extend directly into liquid storage container 14 as does first conduit 15. In either event the other end of second liquid discharge conduit 23 joins with heat exchanger means 25, illustrated as a vaporizer extending the end-to-end length of chamber 11 beneath floor 26 but in thermal association therewith. Alternatively, heat exchange means 25 could be located above the floor at one end of the storage chamber. Although illustrated as one passageway, heat exchanger 25 preferably comprises two sections positioned on either side of the longitudinal centerline of chamber 11, and provides sufficient heat transfer surface area to insure vaporization of the cold liquid continuously flowing thereto. This heat is supplied in large part by the warmer circulating chamber environment gas contacting the outer surfaces of heat exchanger 25.

The discharge end of heat exchanger 25 joins external heat exchanger 27 wherein the cold nitrogen fluid may be superheated by atmospheric heat. The resulting warmed nitrogen gas is returned to the product chamber 11 through conduit 28 communicating with gas expander 29, which is preferably centrally positioned in the upper portion of the chamber near the front end of storage chamber 11. As illustrated the front end of chamber 11 is also the inlet end of spray conduit 21, but the conduit might also be positioned with its discharge end at the front of chamber 11 adjacent expander 29. The latter is preferably a commercially available sliding vane-type air motor with an inlet pressure of about 10-25 psig. operating at 200-1,500 rpm. or greater, but a turbine type expander may be used. Expander 29 is joined by shaft coupling means 30 to fan 31 also positioned near spray conduit 21, and preferably between the liquid storage container 14 and the first spray opening 22a. If the desired operating speed of fan 30 is not suitable for the driving motor 29, speed change by belt drive or gears may be used.

The exhaust gas from expander 31 is preferably directed into chamber 11 through the discharge port 32. Direct discharge through expander port 32 at atmospheric pressure is preferred to obtain maximum pressure drop across the expander. This in turn develops as much shaft power as possible for driving fan 31.

Fan 31 circulates environment gas, including a portion of the cold nitrogen intermittently sprayed from openings 22, across the upper section of chamber 11 over product 12 to the rear end thereof and downwardly to the bottom passages beneath floor 26. The floor structures of commercially employed product storage chambers, e.g. trucks, trailers or railcars, usually comprise wood slats or shaped metal such as channels or corrugations which are spaced apart and shaped to provide adequate structural strength and light weight. These structures also provide spaces for adequate longitudinal gas circulation under the product load. Cold fluid conduit 25 may be supported within such depressions in the floor and thermally insulated therefrom. The thermal insulation should be sealed from the atmosphere to prevent moisture penetration and consequent loss of efficiency. The fan circulates gas from end to end of the chamber 11 floor in passageways formed by such channels or corrugations to recover refrigeration from the liquid-vapor in conduit 25. The thus warmed environment gas rises at the chamber front end and is at least partially recirculated by fan 31 in the aforedescribed manner. A part of this gas may be discharged to the atmosphere through vent 32a to avoid overpressuring in chamber 11.

During the winter the temperature difference between the surrounding atmosphere and the desired refrigeration level within chamber 11 is smaller than in the summer. Accordingly the combined heat exchange capacity of internal unit 25 and external unit 27 may not be required during the winter. In this event passageways 25 may be bypassed and the nitrogen liquid from second discharge conduit 23 diverted through conduit 33 and control valve 34 therein directly to atmospheric heat exchanger 27. Under these circumstances the nitrogen liquid is both vaporized and superheated by atmospheric heat prior to flow through expander 29.

The unexpected advantages of this invention are illustrated in a series of tests as follows.

TEST NO. 1

A 45 foot long stationary railcar having thermal insulation of sufficient quality to limit the heat inleak rate to about 0.07 Btu/hr. .degree.F.-ft..sup.2 was provided with the components illustrated in FIG. 2 and first tested with a 3/4-inch ID by 7/8 -inch OD copper spray header containing twenty-five 1/16 -inch diameter openings longitudinally spaced end-to-end. The spray header was insulated with 1 inch of urethane foam so that the fluid discharged through the openings comprised mostly liquid. The set point for bulb 17 was 0.degree.F. and the cross-sectional area of the chamber was about 86 ft.sup.2. Nitrogen refrigerant was sprayed into the product chamber at an effective flow rate of about 143 lbs./hr./10 ft. average length of chamber through the 25 openings, having a total area of about 0.016. inch.sup.2 per 10 foot length of chamber. The maximum flow rating was about 185 lbs/hr. N.sub.2 /10 ft. average length. After observing that the temperature distribution within the chamber was undesirably uneven, all but five of the spray openings were closed to provide total open area of 0.003 inch.sup.2 /10 ft. average length of chamber. These remaining five openings were located the following distances from the spray conduit inlet end: 3.7, 13.6, 25.9, 36.1 and 44.2 feet. The refrigerant flow rate was thereby reduced so that the refrigerant "on" cycle was lengthened, the spray header cooldown time became a smaller fraction of the "on" cycle, and the upset in temperature distribution due to this cooldown was minimized. Another advantage of the lower refrigerant effective flow rate was lower pressure drop in the spray header which permitted maximum pressure drop (to atmospheric pressure) across the spray openings. This resulted in greater atomization of the liquid nitrogen on discharge through the openings, and more uniform distribution of refrigeration to the product. In addition to the runs with five openings, other runs were made with six openings located the following distances from the spray header inlet end: 7.4, 17.8, 27.7, 34.6, 39.4 and 44.2 feet; and eight openings located the following distances from the spray header inlet end: 7.4, 13.6, 19.9, 25.9, 31.2, 36.1, 40.4 and 44.2 feet. These runs are summarized in Table B and show that comparable or superior temperature distribution was achieved at much lower maximum flow ratings per 10 ft. average length of chamber. ##SPC2##

TEST NO. 2

Five railcars equipped with 1/4 inch-IPS brass pipe uninsulated spray headers and forty-five 1/16-inch diameter holes were converted to copper spray headers 1/2-inch ID X 5/8-inch OD insulated with 1/2 inch thick elastomer foam and provided with ten 1/16-inch diameter openings to test this invention. These railcars were provided with sufficient plastic foam thermal insulation in the walls to limit the heat transfer inleak rate to about 0.07 Btu/hr.-.degree.F.-ft.sup.2, and used to transport and refrigerate fresh produce such as lettuce, plums, cauliflower and avocados. The operating data is summarized in Table C.

TABLE C

max. LN.sub.2 total area of flow rating average number holes - effective per 10 ft. product spray in.sup.2 /10 ft. LN.sub.2 flow lgth. avg. - temperature holes lgth. avg. rate - lb/hr. lbs/hr. deviation .degree.F 45 0.040 1200 470 3-5 10 0.009 270 100 2-4

TEST NO. 3

A 37 foot long chamber of 55 ft.sup.2 cross-sectional area equipped with apparatus similar to FIG. 2 but comprising a tractor-pulled trailer was used to transport fresh lettuce from Blythe, California to Tonawanda, New York over a seven-day period during the month of December with a 35.degree.F. set point. The trailer was provided with sufficient plastic foam thermal insulation to limit the heat transfer inleak rate to about 0.06 Btu/hr.-.degree.F.-ft.sup.2. The thermally insulated spray header had previously included thirty-two 1/16 -inch diameter holes (total area of about 0.025 inch.sup.2 /10 ft. length average) and freeze damage had been experienced on similar trips in the uppermost layers of the lettuce product. In this test the insulated spray header had ten 1/16 -inch diameter openings (total area of about 0.008 inch/10 ft. length average). The as-received product had virtually no freeze damage and the nitrogen consumption was materially reduced, representing a significant improvement over the conventional high flow rate system. The data from these runs is summarized in Table D.

TABLE D

Max. LN.sub.2 Total Area of Flow Rating Average Number Holes - Effective per 10 ft. product spray in.sup.2 /10 ft. LN.sub.2 Flow lgth. avg. - Temperature Holes lgth. avg. Rate - lb/hr. lbs/hr. Deviation .degree.F 32 0.026 850 330 5-10 10 0.008 270 100 3-4

TEST NO. 4

A series of runs were made using a product storage chamber in the form of an insulated truck similar to FIG. 1, about 48 ft.sup.2 cross-sectional area and about 17.5 feet long. The truck was insulated with nominal 3 inch polyurethane foam to provide a heat inleak rate of about 0.05 Btu/hr-.degree.F-ft.sup.2. The product was frozen food and the set points were +10.degree.F. and -5.degree.F. Periodic stops were made to unload the frozen food, first using a conventional uninsulated spray header having ten 1/16 -inch diameter openings (0.018 in.sup.2 /10 ft. length average) and then with another spray header according to this invention with six 3/64 -inch diameter openings (0.006 in.sup.2 /10 ft. length average). The latter spray header consisted of two 57 inch long sections of 1/4-inch IPS brass pipe connected by a 24 inch blank section. The spray header was side-mounted near the chamber ceiling 33 inches from the chamber front end, and the 3/64 -inch diameter openings were located the following distances from the inlet end:

Number Hole Distance (inches) 1 4 2 19 3 34 4 49 5 109 6 136

The data from these runs is summarized in Table E as follows: ##SPC3##

The frozen products refrigerated by each system were delivered in substantially the same satisfactory condition. It is apparent from Table E that the nitrogen spray refrigeration system of this invention permitted a very significant reduction in refrigerant consumption at no loss in performance, indicating that the refrigerant was used more effectively.

TEST NO. 5

Another group of runs was made using the same product chamber as in Test No. 4, but with a spray header having eight 0.038 inch diameter holes (0.005 in.sup.2 /10 ft. length avg). These holes were spaced the following distances from the header inlet end:

Number Hole Distance (inches) 1 6 2 21 3 36 4 47 5 88 6 103 7 118 8 129

The set point was again -5.degree.F. and the product was frozen food, e.g. boxed vegetables, meat and canned fruit juices. The data from these runs is summarized in Table F as follows: ##SPC4##

As before, the frozen products refrigerated by each system was delivered to the customer in satisfactory condition with substantially the same temperature pattern inside the storage chamber. It is apparent from Table F that the nitrogen spray refrigeration system of this invention permitted a very significant reduction in refrigerant consumption at no loss in performance, indicating again that the refrigerant was used more effectively.

Although preferred embodiments of this invention have been described in detail, it is contemplated that modifications of the method and apparatus may be made and that some features may be employed without others, all within the spirit and scope of the invention. For example the invention may be used for stationary refrigeration systems as well as in-transit systems.

Claims

What is claimed is:

1. Apparatus for refrigerating perishable products comprising in combination:

a. a storage chamber for said perishable products having thermal insulation sufficient to limit heat inlead rate below 0.10 Btu/hr-.degree.F-ft.sup.2 and cross-sectional area between 40 and 100 ft.sup.2 ;

b. a thermally insulated container associated with the storage chamber for holding pressurized nitrogen-rich liquid;

c. spray conduit means positioned within the upper portion of said storage chamber and extending substantially the entire length thereof with openings of equivalent diameter less than 0.07 inch spaced along the length for discharging a multiplicity of discrete cold fluid streams into the storage chamber for refrigerating same, said openings having total area of between 0.002 and 0.009 in.sup.2 per 10 ft average length of chamber;

d. fluid discharge conduit means communicating at one end with the liquid container and at the other end with said spray conduit;

e. fluid flow control means comprising a temperature sensing element positioned within said storage chamber, and a control valve operably interposed in said fluid discharge conduit means being connected to said temperature sensing element to be responsive to the storage chamber gas temperature as sensed by such element.

2. Apparatus according to claim 1 in which thermal insulation is provided around said spray conduit means.

3. Apparatus according to claim 1 in which the openings are of about 0.038 inch diameter and circular.

4. Apparatus according to claim 1 in which the spray conduit means comprises multiple sections of 1/4 -inch IPS pipe 57 inches long each having four 0.038 inch diameter circular holes spaced respectively 10, 21, 36 and 51 inches from one end.

5. Apparatus according to claim 1 in which the total area of said openings is between 0.004 and 0.008 inch.sup.2 per 10 foot average length of chamber.

6. Apparatus according to claim 1 in which a fan is positioned within the storage chamber at one end thereof to circulate environment gas.