Spray Refrigeration System For Low Temperature-sensitive Product

Abstract

A system for spray refrigeration of low temperature-sensitive products at optimized slightly above freezing temperature, but avoiding freeze damage at sub-freezing ambient temperatures, by introducing external heat in response to sensing of the ambient temperature.

Description

BACKGROUND OF THE INVENTION

This invention relates to a method of and apparatus for improved intransit spray refrigeration of low temperature-sensitive perishable foods such as fresh meat, leaf vegetables and bananas, using cryogenic fluid.

In the low temperature shipment of fresh perishable food products, temperature controls are usually set at points comfortably above freezing, for example at 40.degree.F. An ample margin-above-freezing allows for nonuniform temperatures throughout all zones of the chamber storage space and takes into account cyclic fluctuations of temperature below the set point (temperature upper limit) provided by the refrigeration system controls. An ample margin also provides a degree of protection against freeze damage due to a reversal of heat flow direction, e.g., against loss of heat through the walls of the storage chamber in the event the ambient air temperature drops below the freezing point of the perishable product.

The ability to control temperature of freeze-sensitive products dependably at or near 40.degree.F. is a relatively recent accomplishment and represents a vast improvement over earlier practice which often permitted temperature excursions to much warmer levels with consequent loss of valuable products. The closer control of shipping temperatures at near 40.degree.F. is an achievement of systems which spray liquefied gas refrigeration directly into the cold product storage chamber. The direct admission of a refrigerant such as liquid nitrogen into the chamber is controlled in response to thermostatic devices sensitive to the chamber temperature as disclosed in U.S. Pat. No. 3,287,935 to J. J. Kane et al. The use of atmosphere-circulating fans in the chamber, and the use of indirect refrigeration applied to the chamber floor are additional improvements aimed at achieving uniform distribution of refrigeration and more uniform temperature throughout the space within the chamber. A further important advance achieved by the direct spray refrigeration, liquefied gas systems is the fast recovery of low temperatures following operation (opening and closing) of the chamber doors. The loss of refrigeration which occurs when the chamber doors are opened can be minimized and localized by the use of load dividing partitions to suppress convection.

Despite the foregoing advances, the use of control settings significantly below 35.degree.-40.degree.F. for low temperature-sensitive products has not been found practical for year-round commercial operation. The exposure of trucks and trailers to severe winter conditions often reduces chamber temperatures well below levels imposed by the on-board refrigeration system and therefore introduces the risk of freeze-damage and loss of product. It should be understood that the temperature level maintained during shipment is heavily influenced by the level to which products are prechilled before shipment. In effect, the product acts as a huge thermal ballast. Despite this ballast effect, exposure of the chamber to severe winter conditions can extract sufficient heat through the chamber walls to reduce the temperature of portions of the contents adjacent the inner walls several degrees below the prechilled level. Such occurrences persist even though thermal insulations for trucks and trailers have been improved significantly in recent years. Therefore, a trailer loaded with prechilled product in a warm region for shipment to markets in a cold region must be maintained during shipment at a temperature level reflecting ample margin-of-protection against freeze damage.

Heating devices are sometimes used on-board trucks and trailers to prevent freeze-damage of product. The heaters are high-capacity systems and thermostatically controlled by chamber temperature such that they operate cyclically or intermittently. The refrigeration system is also high capacity, at least with respect to refrigeration requirements prevailing when ambient temperature is near freezing. It is evident that if the two, opposed high-capacity systems become operative simultaneously, then the refrigeration and heat would be expended one against the other in a wasteful manner. The rate of introduction of refrigeration and heat would bear no relationship to the heat leakage between the storage space and its environment. If refrigeration were supplied by liquefied gas and heat were supplied by a combustible fuel, then the on-board supplies of both refrigerant and fuel would be quickly and needlessly consumed. For this reason, it has been usual practice to set the heater control to be activated at a temperature well below that of the refrigeration system so that no overlap of their operative temperature ranges could occur. The temperature span between the set points of the two systems being necessarily substantial, the combined systems are therefore incapable of maintaining close temperature control of the storage space.

More specifically, according to the prior art the operative temperature range of each system (heating and cooling) is usually fixed by an associated thermostatic device each responsive to chamber temperature and amounts to at least several (e.g., 2-3) degrees. Thus, the refrigeration system may be activated at, say 42.degree.F. and deactivated at 39.degree.F. The heater system may be activated at 32.degree.F. and deactivated at 35.degree.F., thereby leaving 4.degree.F. margin (35.degree. to 39.degree.F.) separating the operating ranges of the two systems. Therefore, the combined systems permit a temperature variation in the storage chamber from 32.degree. to 42.degree.F., or 10.degree.F. change.

Another shortcoming of prior art heating systems for spray refrigerated trailers is their slow response time. When the heater is controlled by storage chamber temperature, then the entire trailer wall structure must first become deeply chilled before the heater is activated. The system must now introduce heat at a high rate sufficient not only to counterbalance the continuing loss of heat through the walls but also to re-warm the walls several degrees to a temperature level to which the produce can remain safely exposed. It is evident that the heater must possess a high capacity in order to temperature-cycle the mass of the walls. It is also evident that the introduction of large doses of heat poses a heat distribution problem and increases the risk of local overheating of product near the zone of heat introduction.

Still another disadvantage of prior art combustion heaters is that they usually employ the product chamber atmosphere as their oxygen source. After extended operation the oxygen of the product atmosphere may be consumed down to the combustion limit and the combustion reaction will be extinguished. This situation is particularly acute when an inert liquefied gas refrigerant system is employed such as liquid nitrogen; simultaneous operation of such heating and refrigeration systems is essentIally impossible. It has been normal practice to manually deactivate the liquefied gas refrigeration system before manually activating a combustion heater systems.

An object of this invention is to provide an improved method of and apparatus for spray refrigeration of freeze sensitive product.

Another object is to provide such method and apparatus in which the product is maintained at significantly closer to optimum temperature than heretofore possible and yet without freeze damage.

Still another object is to provide such method and apparatus that avoids local overheating and damage of perishable product.

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, temperature-sensitive product is supported on a metal base in a thermally insulated closeable storage chamber and refrigerated by overhead spraying a cryogenic fluid having a boiling point at atmospheric pressure below about -20.degree.F. in response to gas temperature monitoring within the storage chamber so as to maintain the storage chamber gas temperature below a predetermined upper limit. In the practice of this invention, the actual upper limit is determined on the basis of the product to be stored, and is preferably about 32.degree.F. for fresh meat, about 38.degree.F. for leaf vegetables such as lettuce and about 56.degree.F. for bananas.

In this method, the ambient temperature outside the storage chamber is sensed and heat introduction is commenced to the storage chamber through the metal base in responSe to such ambient temperature sensing when the ambient temperature drops to within 5.degree.F. of the chamber gas temperature upper limit. Moreover, the heat is introduced at average density of 1-15 watts per square foot of metal base area -- a very low rate as compared to prior art heating systems for intransit refrigeration of freeze-sensitive products. The heat is however introduced at rate higher than the rate at which heat flows through the chamber walls to the ambient air so as to maintain the chamber inner wall at temperature not substantially colder than the sensed chamber gas temperature. Part of this heat is transferred to the sprayed cryogenic fluid. The heat input average density should be at least 1 watt per square foot of metal base area to achieve the desired temperature control in the storage chamber, and less than 15 watts to avoid product loss due to excessive warming and inordinate consumption of refrigeration. In a preferred embodiment, heat is introduced into the storage chamber in at least two increments with the first increment introduced when the ambient temperature is within 5.degree.F. of said chamber gas temperature upper limit and a second heat increment is introduced when the ambient temperature is more than 5.degree.F. below the chamber temperature upper limit.

In the apparatus aspect of the invention, a closeable storage chamber for the products is surrounded by thermal insulation sufficient to reduce heat transfer below 0.1 Btu/hr .times. ft.sup.2 .times. .degree.F., and a thermally insulated container is associated with the storage chamber for storing pressurized liquefied gas having a boiling point at atmospheric pressure below about -20.degree.F., as for example pure nitrogen or nitrogen mixed with oxygen. Spray conduit means are positioned within the upper portion of the storage chamber for discharging a multiplicity of discrete cold fluid streams into the storage chamber to refrigerate the product. Liquefied gas flow control means comprise a gas temperature sensing element positioned within the storage chamber, a fluid discharge conduit between the liquefied gas container and the spray conduit means, a flow control valve in the fluid discharge conduit, and signal transmitting means from the gas temperature sensing element to the control valve for controllably releasing liquefied gas from the container through the spray conduit means into the storage chamber when the sensed temperature is in a predetermined temperature range having a refrigeration release temperature upper limit.

The apparatus also includes a metal base in the floor of the storage chamber extending substantially the entire length and width thereof for supporting the product. A multiplicity of electrical resistance heating elements are positioned in spaced relationship beneath the metal base in heat transfer relation therewith, each extending substantially the entire length of the metal base. Power source means are provided for the electrical resistance heating elements. An ambient air temperature sensing element is positioned outside the storage chamber, and signal transmitting means join the element and the power source for activating at least some of the electrical resistance heating elements when the ambient air temperature drops to within .+-. 5.degree.F. of the refrigeration release temperature upper limit.

It should be understood that the activation temperature of the electrical resistance heating elements is based on the particular requirements of the perishable products to be refrigerated, and may be above or below the refrigeration release temperature upper limit, but in any event within 5.degree.F. of same. The temperature difference must be small in order to minimized the temperature variation in the storage chamber and closely approximate the optimum refrigeration temperature for product to be transported. For example, if fresh beef is to be shipped the storage chamber temperature must in any event be kept above 27.degree.F., the beef freezing point. A preferred refrigeration release temperature upper limit for the refrigerant spray into the storage chamber is 31.degree.F., and the heating element activation temperature is preferably above this temperature but not more than 36.degree.F. and preferably about 32.degree.F. This means that a "thermal overlap" occurs, i.e. heat introduction is commenced when the ambient air is still slightly warmer than the refrigeration release temperature control setting, and this heat is absorbed by the sprayed refrigerant. However, the thermal overlap usually exists for a short time period. The "extra" refrigerant consumed during this period is a minor consideration compared to the improved quality of the beef at destination, relative to the prior art shipment at warmer, less precise temperature level controls. A few degrees of thermal overlap helps to insure positive, close control of chamber temperature with a low rate of heat addition, because it permits the heater control to anticipate a drop in ambient temperature to a level below the refrigeration release temperature control setting. By commencing heat addition slightly in advance of an actual need for heat, a "control lag" is avoided because the metal base will be prewarmed in ample time to prevent an uncontrolled decline in chamber temperature.

As another example, fresh lettuce begins to freeze at about 31.7.degree.F. and a preferred refrigeration release temperature upper limit is 38.degree.F. Experience has shown that storage below this limit but above freezing results in best quality and long shelf life of the delivered product. In this system it is preferred to fix the heating element activation temperature at a lower level for conservation of refrigeration, e.g. 33.degree.F. This heater activation temperature permits a relatively wide margin between the product freezing temperature and the upper limit for storage of such product according to the present invention, e.g. 38.degree.F. In contrast to the fresh beef product embodiment, there may be no thermal overlap because the heating element is only activated when the ambient air temperature drops several degrees below the refrigeration release temperature upper limit. It should be understood that although 38.degree.F. is the preferred upper limit for practicing this invention for leafy vegetables the refrigeration release control may be set at a temperature lower than 38.degree.F. at the discretion of the operator, provided that the on-off range of the control does not extend down to the freezing point of the product. The refrigeration system of this invention is a high rate system capable of overriding the warming effect of the heater system; hence the heater system cannot protect the product if the refrigeration system should operate down to an excessively low temperature.

As a further illustration, bananas are preferably maintained at a significantly higher temperature so that if the refrigeration release temperature upper limit is 56.degree.F. an appropriate heating element activation temperature is 54.degree.F. Thermal overlap is preferred for close control but is not essential.

An especially important aspect of the invention is that the heating is controlled in response to ambient temperature rather than to the temperature of the product storage chamber. If the inside temperature were used for heater control, the chamber thermal insulation would necessarily become deeply chilled before activating the heaters. Under these circumstances the heaters would require sufficient heating capacity to not only replenish heat as rapidly as it escapes through the chamber outer walls but also to rewarm the chamber inner walls and the thermal insulation contiguous thereto. The resultant large dosage of heat would necessarily be quickly and uniformly distributed to avoid warming the product above the desired level. Even more importantly, it would be necessary to terminate the rapid and massive heat introduction at a temperature below the operating range of the spray refrigeration system, else such heat would drastically increase the rate of refrigerant consumption. Because the warming range of a high-capacity heater and the cooling range of the spray refrigeration system could not overlap but must be spaced apart in actual practice, the gross range of temperature control would be too wide for optimum quality and shelf life of many perishable products.

The heater system of this invention senses ambient air temperature and in effect anticipates an increase in temperature difference across the walls of the thermally insulated product storage chamber which would be sufficient to chill the inner wall and insulation to an undesirably low temperature. The heater than adds heat at only a slow rate of 1-15 watts per square foot of metal base area -- needed to sustain the inner wall temperature despite a higher temperature gradient across the wall. The chamber inner walls are maintained at temperature not substantially colder than the sensed chamber gas temperature as fixed by the spray refrigeration system, and the need for a massive influx of heat to rewarm the walls is eliminated.

As previously indicated, the heat is introduced at rate higher than the rate at which heat flows through the chamber walls to the ambient air, with part of the heat absorbed by the sprayed cryogenic fluid. Moreover, the chamber floor assembly has substantial heat storage capacity and continues to dissipate heat to the chamber environment even after the heating may have been terminated by a rise in ambient temperatures. Most of this stored heat is also transferred to the refrigerant. The heat may be transferred by solid conduction through the metal base and by convection to gas circulating to the chamber. For example, lettuce may be refrigerated in the system with a chamber gas temperature upper limit of 38.degree.F. and a heater activation temperature of 33.degree.F. If during storage the ambient air drops to 33.degree.F. the heaters are activated and the heater floor assembly is warmed and causes continued cycling of the refrigeration system. Even through the ambient air rises above perhaps 36.degree.F. so that the heaters are deactivated, the floor assembly will have absorbed sufficient heat to warm the chamber gas to 38.degree.F., thereby activating the cryogenic spray refrigeration system. This relationship exists because of the small 5.degree.F. temperature span between the refrigeration release temperature upper limit and the heater activation temperature, which is necessary to maintain the product within the desired close temperature limits only slightly above the low temperature product damage point.

One important advance of this invention is the type of ambient air temperature sensing element employed. Since the inner wall temperature is fixed and held constant by the spray refrigeration system, it is not necessary to sense the temperature difference between the product chamber and the ambient air. This is fortunate because temperature difference sensors and controllers are expensive and not sufficiently rugged and durable for truck and trailer service. The heater control of this invention may be a simple, rugged ambient temperature sensor as for example a fluid-filled bulb or bimetallic actnator.

Another advantage of the invention is that the use of a low-capacity low-density heating system prevents damage to heat-sensitive materials in the immediate vicinity of the heaters. As will be described below in greater detail, the electric heaters are preferably positioned within closed longitudinal channels beneath the metal base. If heat was rapidly introduced per unit metal base area the temperature within the channels could readily rise to levels which damage both thermal and electrical insulation adjacent to one associated with the heater elements.

Fresh beef may be shipped without freeze damage in the system of this invention using a storage chamber nitrogen spray refrigeration release temperature upper limit of 31.degree.F. when the ambient air is subfreezing. This temperature closely approximates the optimum for storage of fresh beef, and represents a substantial improvement over the 40.degree.F. upper limit of prior art intransit spray refrigeration systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view taken in elevation, of a tractor-trailer assembly incorporating one embodiment of the invention.

FIG. 2 is a schematic flowsheet showing the fluid and electric interconnections of a refrigerant spray-electric heating system suitable for use in the FIG. 1 assembly.

FIG. 3 shows the three-phase delta electric circuit preferred for the activation of the electric resistance heating elements.

FIG. 4 is a cross-section transverse view taken in elevation on an enlarged scale, of part of a preferred floor assembly of a product storage chamber.

FIG. 5 is an isometric view looking downwardly on an aluminum extrusion with a positioned electric resistance heating element suitable for fabrication of the FIG. 4 floor assembly, and

FIG. 6 is a schematic flowsheet of the fluid and electric interconnections for deactivation of the electric heating system in the event of an inoperative refrigerant spray system.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 shows trailer 11 attached to tractor 12 and supporting perishable product storage chamber 13 composed of multiple compartments 13a, 13b, 13c and 13d. These compartments are separated from each other by transverse load dividing partitions 14a-b, 14b-c and 14c-d which are retractable as for example by rolling up to the ceiling. Each compartment may be provided with a separate refrigeration system independently controlled by the temperature within the specific compartment. It should be recognized however that the transverse partitions and the separate refrigeration systems are not essential to the practice of the invention; only one compartment and one refrigeration system is needed.

Product chamber 13 may be constructed with an outer casing 15 of material such as corrugated steel or aluminum, rear doors 15a and metal heat conductive base 16. Inner walls 17 may be formed of sheet metal fiber reinforced plastic or plyboard, and thermal insulation 18 between outer casing 15 and inner wall 17 is a suitable low thermal conductive material such as foamed plastic or glass fiber.

As previously indicated, the thermal insulation 18 should be sufficient to reduce heat transfer below 0.1 Btu/hr .times. ft.sup.2 .times. .degree.F. For example, about 3 inches of polyurethane insulation of 0.0195 Btu/hr .times. ft.sup.2 .times. .degree.F/ft avg. thermal conductivity is suitable for a temperature difference, ambient to chamber, of 80.degree.F. or lower. It will be recognized by one skilled in the art that the aforementioned heat transfer upper limit may be achieved by employing a relatively thin layer of highly efficient insulation or a relatively thick layer of less efficient (and less costly) thermal insulation. Thermal insulation effectiveness is an important aspect of this invention because it directly affects the heating requirements to obtain the aforestated objects. If the thermal insulation were 100 percent effective, heating would not be required and on the other hand if the insulation were extremely inefficient the spray refrigeration system would not perform its function at warm ambient and the heating requirements would be prohibitively high at low ambient temperature. The thermal insulation effectiveness of below 0.1 Btu/hr .times. ft.sup.2 .times. .degree.F. represents an appropriate balance between these considerations and facilitates the improved performance of the present system as compared to prior art spray refrigeration systems.

The refrigeration system includes one or more liquefied gas containers 19, control boxes 20 and 21 mounted on the storage chamber 13 at convenient locations, and overhead spray conduit means 22, for example a multiple orifice nozzle. At least one such spray conduit 22a, 22b, 22c and 22d is provided in the upper portion of each respective compartment 13a, 13b, 13c and 13d. These spray conduits are operated in response to associated gas temperature sensing elements 23a, 23b, 23c and 23d in the compartments.

The heating system includes a multiplicity of electrical resistance elements 24 mounted beneath the metal base 16 in heat transfer relation therewith and preferably extending substantially the length of the base. Electric generator 25 in the tractor 12 is joined to elements 24 by electrical supply conductors 26 terminating in receptacle 27 mounted on the front of chamber 13. Preferably an alternative power source (not illustrated) is provided for use when the trailer 11 is not adjacent tractor 12, as for example at the loading dock, and second receptacle 28 is attached to the rear of product chamber 13 for connection to a dockside electrical supply system. It will be understood that FIG. 1 merely shows the layout of the major components of the system; interconnections are illustrated in the ensuing figures.

Referring now to FIG. 2, the interconnection of the refrigerant spray and electric heating components and their function is illustrated and will be described in detail. The liquefied gas container 19 is filled from a suitable source of such refrigerant through connection 20 and preferably sealed at saturated condition and at temperatures corresponding to a vapor pressure above 10 psig. with the entire liquid and vapor substantially in equilibrium, e.g. 20 psig. and -298 .degree.F for a 84 percent N.sub.2 - 16 % O.sub.2 by volume mixture. Under these conditions, the vapor pressure is sufficient to dispense the entire contents of the container 19 without external heat. Alternatively the cryogenic liquid may be stored in container 19 under non-saturated conditions, i.e. in the subcooled state. Under these conditions, it would be normal practice 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 well-known 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).

When refrigeration is demanded in any given compartment of the storage chamber 13, liquid is withdrawn from container 19 through discharge manifold 29 and thereafter through the appropriate branch conduit, e.g. 30a communicating with compartment 13a. The function and operation of each refrigeration supply system for compartments 13a, 13b, 13c and 13d is essentially identical and in the interest of simplicity only one such system will be described. Assuming that the temperature in storage compartment 13a rises to the predetermined refrigeration release temperature as sensed by element 23a, e.g. a fluid filled bulb, an electrical or pneumatic signal representative of such condition is transmitted through conductor 31a to pneumatic relay 32a. The latter receives gas at the pressure of container 19 through an instrument supply system including vaporizer 33 in an instrument supply conduit 34, and branch conduit 35a. The refrigeration deficient signal through conductor 31a causes relay 32a to release instrument gas pressure from instrument supply branch conduit 35a into instrument gas control conduit 36a connected to pneumatically controlled valve 37a in refrigeration supply branch conduit 30a. This opens valve 37a and allows refrigerant to flow through conduit 30a and spray header 22a for example mounted on the ceiling of refrigerant-deficient compartment 13a.

Pure liquid nitrogen is a suitable refrigerant for use with many perishable products. If one compartment of chamber 13 is to be entered while another is maintained under automatic control of its refrigerant system as heretofore described, then the preferred refrigerant is a mixture of oxygen and liquid nitrogen of such composition as to produce a breathable atmosphere in the refrigerated compartment. In the latter case the oxygen content of the liquid is preferably at least 15 percent by volume but not in excess of about 18 percent. In general, liquid refrigerant of higher oxygen concentration within the foregoing range will require more effective thermal insulation systems for refrigerant storage container 19 in order to avoid oxygen enrichment of the stored refrigerant to level substantially higher than 21 percent. The use of oxygen-nitrogen mixtures as the refrigerant is further contingent upon compatability of such mixtures with the product which is stored in the compartment. Whereas the preservation of some products such as fresh beef is enhanced by relatively high oxygen atmospheres as for example 12 percent oxygen by volume, other products such as leafy vegetables are best stored in an atmosphere produced with a pure nitrogen refrigerant. Regardless of the refrigerant employed, the refrigeration system for a product storage compartment should be deactivated before the compartment is entered. This can be accomplished by closing the instrument gas supply valve 34a in conduit 35a.

As previously indicated, the heating system includes a multiplicity of electrical resistance heating elements positioned in heat transfer relation with the metal base 16 of the product storage chamber. As illustrated in FIG. 2, the electrical supply for the heaters is three-phase power supplied from the tractor generator 25 or from a dockside electrical system through one of the receptacles 27 and 28. The heaters are connected in a delta arrangement. For purposes of illustration only one resistance element and one switch is shown per phase but in actual practice each phase may contain a plurality of resistance elements and each switch may contain three contacts permitting one or more elements to be energized simultaneously in each phase of the circuit. In such an arrangement, operation of any switch will maintain a balanced load on the three-phase power supply. A preferred multiple resistance arrangement for the heaters will be described hereinafter in connection with FIG. 3, but for purposes of understanding FIG. 2 it is sufficient to recognize that the heaters are arranged in three separate sets with the heaters of a particular set being equally distributed among the three phases. Each set of heaters is controlled by a separate thermostatic switch 38a, 38 b or 38c. Whereas the switches are illustrated as single-pole devices, it should be understood that each is a three-phase switch so that each switch comprises three independent contacts, one for each phase of the circuit. Each of the switches is independently actuated by an ambient air temperature sensing element 40a, 40 b or 40c outside the storage chamber. The actuation system comprises relays 41a, 41b and 41c joined to ambient air temperature sensing bulbs 40a, 40b and 40 c respectively by signal receiving means 42a, 42b and 42c, and joined to switches 38a, 38b and 38c respectively by switch actuating means 43a, 43b and 43c.

Each of switches 38a, 38b and 38c is preset to close at a different minimum temperature sensed by its corresponding element, e.g. fluid filled bulbs or bimetal switch actuators 40a, 40b or 40c exposed to ambient temperature. For example, switch 38a may be set to close when the ambient air temperature drops to a first level of 30.degree.F. Electric power will be thereby supplied to activate a first set of resistance heaters 44 in the chamber floor. Switch 38b is set to close when the temperature sensed by bulb 40b falls to a second lower level as for example 15.degree.F. and the power is furnished to a second set of heaters 45. Similarly, switch 38c close when the temperature sensed by bulb 40c falls to a third level below the second temperature for example -5.degree.F. thereby supply power to a third set of heaters 46 in the trailer floor. This stepwise control of heater addition to the storage chamber 13 achieves maximum simplicity in the instrumentation and electrical components, yet introduces heat incrementally in accordance with the temperature difference prevailing between the product storage space and the colder ambient atmosphere. The operation of the heaters is continuous once the power supply switch in a given circuit has been closed In other words, operation is not modulated or cycled in response to the temperature variations within the trailer.

Since each switch remains closed at all temperatures below its operating point, it is evident that a set of resistance elements which is activated at a relatively high temperature level will remain in operation when a set is activated at a lower temperature level. Thus, the total heat supply by the heater system will be the sum of the heater ratings of all heater sets which have been activated at and above the prevailing temperature. For example, at 14.degree.F. in the aforementioned illustration, both the heater sets 44 and 45 controlled by switches 40a and 40b will be operative.

The precise number of heaters and their individual wattage is subject to choice. It is preferred that no fewer than three heaters be in operation together in order that the heat input will be reasonably well-distributed over the chamber metal base. When three-phase power is used, the number of heaters should preferably also be a multiple of three so that the heating load can be balanced. The total wattage supplied by the heating system will be dependent upon the severity of winter conditions to be encountered, upon the effectiveness of the storage chamber insulation system, and upon the margin between the temperature level maintained by the refrigeration system and the minimum permissible level which avoids damage to the product. The total heat load to be provided in a conventionally-sized trailer about 45 feet long, 8 feet wide and 8 feet high may be on the order of 3,000 watts with all sets of resistance elements in operation. A suitable arrangement of heaters for such a trailer may comprise three elements in set 44 and six elements each in sets 45 and 45. Thus, the fifteen elements may each be rated at 200 watts. If the elements extend substantially the full length of a 45 foot long trailer, then heater elements providing 5 watts per foot of element length are suitable. Such elements of each set are spaced at equal distances in the transverse direction across the chamber metal base, e.g. the three elements of set 44 may be for example at about 30 inch intervals and the five elements of sets 45 and 46 are each at approximately 17 1/2 inch intervals.

The preferred method for connecting the heater elements in the three-phase, delta scheme is shown in FIG. 3. The first set 44 of heaters consists of three resistance elements 44, 44b and 44c, one of which is connected in each phase of the delta circuit. It is also seen that each element 44a, 44b and 44c is connected respectively in series with one of contacts 48a, 48b and 48c, all contained in switch 38a of FIG. 2.

The second set 45 of resistance elements consists of six elements 45a- f, two of which are provided in each phase of the delta connection in a similar manner to first set 44. Two heater elements in each phase are wired in parallel and are connected in series with one of contacts 49a, 49b, and 49c contained in switch 38b. The third set 46 of heaters also consists of six resistance elements 46a-f which are interconnected into the delta circuit in the same manner as second set 45. Resistance elements 46a-f are paired, one pair to each phase, and are controlled by contacts 50a, 50 b and 50c in switch 38c. By distributing the resistance elements comprising a set equally among the three phases of the power supply, the load on the supply will be balanced at all times.

The first set 44 of heaters which are controlled by switch 38a are placed in operation at the warmest temperature level sensed by elements 40a, 40 b and 40c. Because the temperature difference across the storage chamber walls has the least value when first heater set 44 is activated, loss of heat from the storage chamber which must be overcome by these heaters is at a comparatively slow rate. For this reason, only three heater elements are provided in first set 44. In the other heater sets 45 and 46 which are activated at substantially lower ambient temperatures, the heat loss which they must replace is relatively greater and six elements are provided in each of these sets.

The product 59a is supported on or in low conductive members as for example wood pallets 59b, which in turn are positioned on the upper surface of metal base 16. Alternatively and if a convenient form as for example beef quarters, the product 59a may be suspended in the chamber gas space. In any event the product should not be placed directly against heat conductive metal base 16 so as to avoid excessive warming of the bottom layer. Heat introduced to the chamber by the electrical resistance elements is transferred to gas continuously circulating in contact with metal base 16 and the warmed gas transfers the heat to the product by contact therewith. The chamber gas flow may be natural circulation supplemented by the refrigeration spray pressure driving force (when the refrigeration system is operating), or a fan may be positioned at the upper end of the chamber, e.g. the front of the FIG. 1 trailer-chamber, to insure the desired flow pattern.

FIG. 4 illustrates a preferred floor assembly supported on transverse beams 52 which are attached to the frame or chassis of the trailer 11 and are usually steel structural members. Outer casing 15 of the product storage chamber 13 rests on these beams with spaced inner wall 17 and a suitable thermal insulating material 18 therebetween. Metal heat conductive base 16 is positioned on inner wall 17 and comprises a multiplicity of panel elements 53, 54 and 55 which for example may be produced a aluminum extrusions extending the entire length of the trailer and welded to each other as at 56. The extrusion includes longitudinal inverted T- sections 57 having lower ends attached to the bottom side of metal base 16 to achieve maximum strength and rigidity with minimum weight and utilization of material. Adjacent inverted T- sections 57 form open channels 58 extending the entire length of the metal base 16, and the electrical resistance heater elements 24 are preferably positioned within such channels. By way of example and using 15 elements as previously described, 54 channels may be provided in the floor assembly and elements 24 may be transversely spaced between every fourth pair of adjacent channels as illustrated, thereby providing uniform distribution of heating elements over the area of the metal base. The fifteen electrical resistance elements may also be grouped electrically as illustrated in FIG. 2 with three elements in first set 44, six elements in second set 45 and six elements in third set 46.

FIG. 5 illustrates the assembly of an electrical resistance heater package which may comprise the unit 24 placed between adjacent longitudinal channels as described in connection with fIG. 4. Aluminum extrusion 59 is slightly narrower than the space between the webs of the FIG. 4 T- sections 57 and upwardly bent thin flange members 60 are an integral part of the extrusion 59. Heater strip 61 comprises a plastic sheet with metal wire embedded therein and leads 62 extending from opposite ends. Strip 61 is installed within extrusion 59 underneath flange members 60, after which the latter are bent downward (as shown by arrows and broken line) to hold the heater strip 61 intact within the channel and to protect the strip against mechanical damage. After assembly the extrusion is inserted within an appropriate channel and the leads 62 are connected into the circuit as for example illustrated in FIG. 3. The heater strip is of any suitable form available in a low heat rating per unit length, preferably in the range of 1-15 watts per foot length. For example, in the FIG. 3 embodiment constantan wire embedded in a fiber-reinforced silicone plastic sheet was used and provides an average rating of 5 watts per foot length.

While three-phase power has been described in detail, it will be understood that single-phase current could be employed. A single feeder wire would be provided and would supply several branch circuits, one circuit for each set of electrical resistance elements installed in the chamber floor. Each branch circuit would be controlled by one of the thermostatic switches 38a-c shown in FIG. 2. Similarly, the voltage of the power supply is subject to choice, it being necessary to choose the heaters to suit the voltage. For example, 120 volts D.C. would be suitable. One advantage of a three-phase, 220-volt supply is that line protective devices (breakers or fuses) have low amperage ratings and are readily available at low cost.

Another modification of the invention is shown in FIG. 6 which duplicates in part the components of FIG. 2 -- a full representation of all components being unnecessary to an understanding of the modification. In FIG. 6, a fluid pressure conductor 63 is branched from the control instrument fluid supply conduit 34 downstream of valve 64 and upstream of the manifold 65 serving the individual relays, e.g. 35a. The fluid pressure conductor 63 leads to relay 66 joined by actuator means 67 to electrical switch 68 located in the electrical feeder wire 69a to the heater circuits. Relay 66 is set to open switch 68 in the event the pressure of the instrument control fluid from the refrigeration system drops to an intolerably low level. In such event, the effectiveness of the refrigeration system would be impaired or lost and the heater system would be deactivated to prevent continued warming of the product. Loss of pressure of the control fluid could be due to inadvertent closure of, or failure to open valve 64. Alternatively, it could signal a loss of pressure in refrigerant storage container 19, due to inadvertent venting of vapor therefrom, or to failure of an associated pressure-building system, or to depletion of refrigerant. If desired, an additional set of contacts in relay 66 can be furnished to actuate an audible or visual warning device to alert a person in attendance.

A series of tests using an externally heated nitrogen spray refrigeration system similar to that previously described and illustrated in FIGS. 1, 2, 4 and 5 demonstrated the remarkably close-to-optimum temperature achievable by this invention. The floor was formed of extruded aluminum sections about 1.25 inch wide. The spray refrigeration control components were set for a 34.degree.F. chamber gas temperature upper limit, and all electrical resistance heating elements were activated to furnish 3,000 watts at an average heat density of about 8.3 watts per square foot of extruded aluminum metal base to a 45 ft. long .times. 8 ft. wide .times. 8 ft. high chamber trailer with about 3 1/2 inches (average) of polyurethane thermal insulation to reduce heat transfer below about 0.07 Btu/hr .times. ft.sup.2 .times. .degree.F.

Two temperature measurements were made of the aluminum base top surface, one directly over an electrical resistance heating element and another measurement midway between adjacent heating elements in the transverse direction of the base. Another temperature measurement was made in the chamber gas space three inches above the aluminum base top surface, and the fourth measurement was six inches below the chamber ceiling. With all three sets of heating elements activated, the metal base temperature rose to 66.degree.F. above the heater element and to 61.degree.F. midway between heater locations. It was thus apparent that the base temperature was quite uniform, demonstrating the excellent transverse distribution of heat obtained through the highly conductive aluminum panels. After steady-state conditions were obtained in the chamber, the gas temperature near the ceiling varied with time only about 1.degree.F. from the refrigeration temperature upper limit control point, i.e. between 34.degree.F. and 35.degree.F. The temperature three inches above the aluminum base was found to be only about 1.degree. higher than the near-to-ceiling temperature at the corresponding time. Stated otherwise, the 30.degree. temperature rise of the metal base top surface was virtually undetectable a distance of three inches above the floor. This means that with the product supported in pallets or carts and not in direct contact with the floor, the product temperature will not be significantly affected by the relatively high temperature of the metal base, even at maximum heat input. This heat will instead by uniformly distributed to the product by gas circulating in contact with the heated floor.

Although preferred embodiments have been described in detail, it will be appreciated that other embodiments are contemplated along with modifications of the disclosed features, as being within the scope of the invention.

Claims

What is claimed is:

1. In a method for the intransit refrigeration of freeze-sensitive products supported on a metal base in a thermally insulated closeable storage chamber by over head spraying a cryogenic fluid having a boiling point at atmospheric pressure below about -20.degree. F. in response to gas temperature monitoring within the storage chamber so as to maintain the storage chamber gas temperature below a chamber temperature predetermined upper limit, the improvement comprising: sensing the ambient temperature outside said storage chamber and commencing the introduction of heat into said storage chamber through said metal base in response to such ambient temperature sensing when said ambient temperature is within 5.degree.F. of said chamber gas temperature upper limit and at average density of 1-15 watts per square foot of metal base area and at rate higher than the rate at which heat flows through the chamber walls to the ambient air so as to maintain the chamber inner wall at temperature not substantially colder than the sensed chamber gas temperature, and transferring part of such heat to the sprayed cryogenic fluid.

2. A method according to claim 1 wherein heat is introduced into said storage chamber in at least two increments with the first heat increment introduced when said ambient temperature is within 5.degree.F of said chamber gas temperature upper limit and a second heat increment introduced when said ambient temperature is more than 5.degree.F below said chamber temperature upper limit.

3. A method according to claim 1 wherein heat is introduced to said storage chamber when the ambient temperature is slightly above the chamber temperature upper limit.

4. A method according to claim 1 for the intransit refrigeration of fresh meat, wherein the chamber temperature upper limit is 32.degree.F. and heat is introduced to said storage chamber when the ambient temperature is below 30.degree. F.

5. A method according to claim 1 for the intransit refrigeration of leaf vegetables, wherein the chamber temperature upper limit is 38.degree.F. and heat is introduced to said storage chamber when the ambient temperature is below 33.degree.F.

6. A method according to claim 1 for the intransit refrigeration of bananas, wherein the chamber temperature upper limit is 56.degree.F. and heat is introduced to said storage chamber when the ambient temperature is below 54.degree.F.

7. Apparatus for the intransit refrigeration of freeze-sensitive products comprising in combination:

a. a closeable storage chamber for said products surrounded by thermal insulation sufficient to reduce heat transfer below 0.1 Btu/hr .times. ft.sup.2 .times. .degree. F.;

b. a thermally insulated container associated with the storage chamber for storing pressurized liquefied gas having a boiling point at atmospheric pressure below about -20.degree. F.;

c. spray conduit means positioned within the upper portion of said storage chamber for discharging a multiplicity of discrete cold fluid streams into the storage chamber;

d. liquefied gas flow control means comprising a gas temperature sensing element positioned within said storage chamber, a fluid discharge conduit between the liquefied gas container (b) and spray conduit means (c), a flow control valve in said fluid discharge conduit, and signal transmitting means from said gas temperature sensing element to said control valve for controllably releasing liquefied gas from said container through said spray conduit means into said storage chamber when the sensed temperature is in a predetermined range with a refrigeration release temperature upper limit;

e. a metal base in the floor of said storage chamber extending substantially the entire length and width thereof for supporting the product;

f. a multiplicity of electrical resistance heating elements positioned in spaced relationship beneath said metal base (e) in heat transfer relation therewith each extending the entire length of said metal base;

g. power source means for said electrical resistance heating elements;

h. an ambient air temperature sensing element outside said storage chamber and signal transmitting means from said element to said power source for activating at least some of said electrical resistance heating elements when said ambient air temperature drops to within 5.degree.F. of said refrigeration release temperature.

8. Apparatus according to claim 7 with a multiplicity of ambient air temperature sensing elements, a multiplicity of ambient air temperature sensing elements, a multiplicity of signal transmitting means each joined to a different temperature sensing element and arranged with electrical resistance heating elements such that a first set of heating elements are activated when the sensed ambient air drops to a first temperature and a second set of heating elements are activated when the sensed ambient air drops to a second temperature below said first temperature.

9. Apparatus according to claim 8 with three ambient air temperature sensing elements and three set of heating elements connected such that the first set of heating elements is activated when the sensed ambient air drops to 30.degree.F., the second set of heating elements is additionally activated when the ambient air temperature drops to 15.degree.F., and the third set of heating elements is still additionally activated when the ambient air temperature drops to -5.degree.F.

10. Apparatus according to claim 7 wherein the electric power source is three phase and the electrical resistance heating elements are connected in a delta arrangement.

11. Apparatus according to claim 7 wherein the electric power source is three phase, with a multiple of three electrical resistance heating elements.

12. Apparatus according to claim 7 wherein said metal base is supported above the inner wall of the storage chamber by a multiplicity of transversely spaced metal members extending the entire length of said chamber, and said electrical resistance heating elements are located between some adjacent pairs of longitudinal members in contiguous relation therewith.

13. Apparatus according to claim 12 wherein a multiplicity of longitudinally aligned aluminum extrusions comprise said metal base and transversely spaced metal members, each extrusion having an upper panel section with adjacent panel edges longitudinally bonded to each other, and also having longitudinally extending lower sections of inverted T- cross-section and bottom end attached to the inner wall of said storage chamber.