EP4466503A1

EP4466503A1 - Supplemental refrigeration using nitrogen

Info

Publication number: EP4466503A1
Application number: EP23707581.7A
Authority: EP
Inventors: John M. GIRARD; Richard A. Novak; Balazs Hunek; Sameer H. ISRANI; Stephen A. Mccormick
Original assignee: Praxair Technology Inc
Current assignee: Praxair Technology Inc
Priority date: 2022-01-21
Filing date: 2023-01-19
Publication date: 2024-11-27
Also published as: WO2023141484A1; CA3253596A1; CN118591710A; MX2024009012A; US20250137693A1; AU2023209864A1

Abstract

Disclosed are methodology and apparatus useful to employ liquid nitrogen cryogen to boost on demand the refrigeration capacity of a freezer or similar device in which refrigeration is provided by a mechanical refrigeration circuit.

Description

SUPPLEMENTAL REFRIGERATION USING NITROGEN

Field of the Invention

The present invention relates to refrigeration (that is, cooling, which may or may not be to the point of freezing) of products such as food products, within an enclosure equipped to provide a chilled atmosphere with which the product to be refrigerated is contacted within the enclosure.

Background of the Invention

Refrigeration apparatus, by which is meant equipment including chillers and freezers, often employ what is known as mechanical refrigeration systems to establish a temperature of the atmosphere inside the equipment that is at or below a given desired level. Product to be cooled or frozen is placed within the equipment and is exposed to the atmosphere so that the product is cooled or frozen, depending on the temperatures of the product and of the atmosphere, and depending on the length of time that the product is exposed to the atmosphere. The present invention provides ways to increase the usefulness of refrigeration apparatus of this type, wherein product is cooled or frozen inside the equipment.

Brief Summary of the Invention

One aspect of the present invention is a method of modifying refrigeration apparatus, comprising providing refrigeration apparatus to be modified that comprises an enclosure enclosing an atmosphere, and that comprises a mechanical refrigeration circuit comprising within the enclosure an evaporator having a heat exchange surface in contact within the enclosure with the atmosphere, the mechanical refrigeration circuit also comprising a compressor, a condenser, and an expansion device, which are connected together with the evaporator in fluid communication so that circulation of a refrigerant through the circuit cools the heat exchange surface and can thereby cool the atmosphere within said enclosure; the refrigeration apparatus also including a refrigeration impeller that circulates atmosphere into contact with said heat exchange surface; and modifying the refrigeration apparatus by adding a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which can be coupled to a vent that is open to the atmosphere outside the enclosure, and positioning the supplemental heat exchanger so that atmosphere that has been cooled by contact with the heat exchange surface of the mechanical refrigeration circuit in the enclosure can be flowed in heat exchange contact with the supplemental heat exchanger, and adding control mechanism that enables liquid nitrogen to flow from said source into the supplemental heat exchanger to further cool, by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, said atmosphere that has been cooled by heat exchange with said mechanical refrigeration circuit, and that enables said liquid nitrogen to flow into the supplemental heat exchanger only when the cooling of the atmosphere in the enclosure that is provided by said mechanical refrigeration circuit without said further cooling is at the maximum that can be provided by said circuit or is within a preset percentage below said maximum.

Another embodiment of the present invention is the aforementioned method which further comprises providing a capability for defrosting, by: modifying the refrigeration apparatus by adding a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, and adding control mechanism that enables the flow of nitrogen to be alternated between a first mode in which nitrogen can flow from the outlet of the supplemental heat exchanger into said recycle line and not to said vent and nitrogen from said recycle line that has been heated by said heater and not liquid nitrogen from said source thereof can flow into the inlet of said supplemental heat exchanger and the nitrogen from said recycle line can be recycled into said supplemental heat exchanger repeatedly, and a second mode in which nitrogen can flow from the outlet of said supplemental heat exchanger to said vent and not into said recycle line, and liquid nitrogen from said source and not nitrogen from said recycle line can flow into the inlet of said supplemental heat exchanger.

Another embodiment of the present invention which comprises providing a capability for defrosting is a method of modifying refrigeration apparatus, comprising providing refrigeration apparatus to be modified that comprises an enclosure enclosing an atmosphere, and that comprises a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which can be coupled to a vent that is open to the atmosphere outside the enclosure, wherein the refrigeration apparatus comprises control mechanism that controllably enables liquid nitrogen to flow from said source into the supplemental heat exchanger to cool said atmosphere by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, and that controls the rate of flow of said liquid nitrogen into the supplemental heat exchanger; modifying the refrigeration apparatus by adding a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, and adding control mechanism that enables the flow of nitrogen to be alternated between a first mode in which nitrogen can flow from the outlet of the supplemental heat exchanger into said recycle line and not to said vent and nitrogen from said recycle line that has been heated by said heater and not liquid nitrogen from said source thereof can flow into the inlet of said supplemental heat exchanger and the nitrogen from said recycle line can be recycled into said supplemental heat exchanger repeatedly, and a second mode in which nitrogen can flow from the outlet of said supplemental heat exchanger to said vent and not into said recycle line, and liquid nitrogen from said source and not nitrogen from said recycle line can flow into the inlet of said supplemental heat exchanger. Another aspect of the present invention comprises a method of refrigerating a product, comprising providing the product in an enclosure that contains an atmosphere, wherein a refrigeration apparatus is present that comprises a mechanical refrigeration circuit comprising within the enclosure an evaporator having a heat exchange surface in contact within the interior space with the atmosphere, the mechanical refrigeration circuit also comprising a compressor, a condenser, and an expansion device, which are connected together with the evaporator in fluid communication so that circulation of a refrigerant through the circuit cools the heat exchange surface and can thereby cool the atmosphere within said enclosure; the refrigeration apparatus also including a refrigeration impeller that circulates atmosphere into contact with said heat exchange surface; and wherein a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which is open to the atmosphere outside the refrigeration apparatus is positioned so that atmosphere that has been cooled by contact with the heat exchange surface of the mechanical refrigeration circuit can be flowed in heat exchange contact with the supplemental heat exchanger, and cooling atmosphere in the enclosure by heat exchange with the heat exchange surface of the mechanical refrigeration circuit; and controllably flowing liquid nitrogen into the supplemental heat exchanger to further cool, by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, said atmosphere that has been cooled by heat exchange with said mechanical refrigeration circuit, wherein said liquid nitrogen is flowed into the supplemental heat exchanger only when the cooling of the atmosphere in the enclosure that is provided by said mechanical refrigeration circuit without said further cooling at the maximum that can be provided by said circuit or is within a preset percentage below said maximum, whereby said product is refrigerated by contact with said atmosphere.

Another embodiment of the present invention is the aforementioned method which further comprises a capability for defrosting, by: alternatingly passing through said supplemental heat exchanger liquid nitrogen from said source thereof to further cool said atmosphere within said enclosure, and heated gaseous nitrogen from a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, wherein said heated nitrogen flows repeatedly through said recycle line and melts ice formed on a surface of said supplemental heat exchanger.

Another embodiment of the present invention which comprises providing a capability for defrosting, is a method of refrigerating a product, comprising providing the product in an enclosure that contains an atmosphere, providing a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which can be coupled to a vent that is open to the atmosphere outside the refrigeration apparatus, wherein the atmosphere can be cooled by heat exchange contact with the supplemental heat exchanger, and cooling atmosphere in the enclosure by heat exchange with the supplemental heat exchanger; controllably flowing liquid nitrogen into the supplemental heat exchanger to cool said atmosphere by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, and alternatingly passing through said supplemental heat exchanger liquid nitrogen from said source thereof to further cool said atmosphere within said enclosure, and heated gaseous nitrogen from a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, wherein said heated nitrogen flows repeatedly through said recycle line into said supplemental heat exchanger and melts ice formed on a surface of said supplemental heat exchanger, whereby said product is refrigerated by contact with said atmosphere. Yet another aspect of the present invention is apparatus for refrigerating a product, comprising an enclosure that contains an atmosphere, refrigeration apparatus that comprises a mechanical refrigeration circuit comprising within the enclosure an evaporator having a heat exchange surface in contact within the enclosure with the atmosphere, the mechanical refrigeration circuit also comprising a compressor, a condenser, and an expansion device, which are connected together with the evaporator in fluid communication so that circulation of a refrigerant through the circuit cools the heat exchange surface and can thereby cool the atmosphere within said enclosure; the refrigeration apparatus also including a refrigeration impeller that circulates atmosphere into contact with said heat exchange surface; and a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which can be coupled to a vent that is open to the atmosphere outside the enclosure, wherein the supplemental heat exchanger is coupled to the refrigeration apparatus so that atmosphere that has been cooled by contact with the heat exchange surface of the mechanical refrigeration circuit can be flowed in heat exchange contact with the supplemental heat exchanger, and control mechanism that enables liquid nitrogen to flow from said source into the supplemental heat exchanger to further cool, by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, said atmosphere that has been cooled by heat exchange with said mechanical refrigeration circuit, and that enables said flow of liquid nitrogen into the supplemental heat exchanger only when the cooling of the atmosphere in the enclosure that is provided by said mechanical refrigeration circuit without said further cooling is at the maximum that can be provided by said circuit or is within a preset percentage below said maximum.

Another embodiment of the present invention is the aforementioned apparatus which further comprises a capability for defrosting, comprising a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, and control mechanism that enables the flow of nitrogen to be alternated between a first mode in which nitrogen can flow from the outlet of the supplemental heat exchanger into said recycle line and not to said vent, and nitrogen from said recycle line that has been heated by said heater and not liquid nitrogen from said source thereof can flow into the inlet of said supplemental heat exchanger and the nitrogen from said recycle line can be recycled into said supplemental heat exchanger repeatedly, and a second mode in which nitrogen can flow from the outlet of said supplemental heat exchanger to said vent and not into said recycle line, and liquid nitrogen from said source and not nitrogen from said recycle line can flow into the inlet of said supplemental heat exchanger.

Another embodiment of the present invention which comprises a capability for defrosting is apparatus for refrigerating a product, comprising an enclosure that contains an atmosphere, a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which can be coupled to a vent that is open to the atmosphere outside the enclosure, wherein atmosphere in the enclosure can be cooled by heat exchange contact with the supplemental heat exchanger, and control mechanism that controllably enables liquid nitrogen to flow from said source into the supplemental heat exchanger to further cool said atmosphere by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, and that controllably enables said flow of liquid nitrogen into the supplemental heat exchanger, and a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, and control mechanism that enables the flow of nitrogen to be alternated between a first mode in which nitrogen can flow from the outlet of the supplemental heat exchanger into said recycle line and not to said vent, and nitrogen from said recycle line that has been heated by said heater and not liquid nitrogen from said source thereof can flow into the inlet of said supplemental heat exchanger and the nitrogen from said recycle line can be recycled into said supplemental heat exchanger repeatedly, and a second mode in which nitrogen can flow from the outlet of said supplemental heat exchanger to said vent and not into said recycle line, and liquid nitrogen from said source and not nitrogen from said recycle line can flow into the inlet of said supplemental heat exchanger.

Some preferred embodiments utilize (a) control mechanism that enables liquid nitrogen to flow into the supplemental heat exchanger and that controls said flow by monitoring the pressure of liquid nitrogen flowing from said source, or (b) control mechanism that enables liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger, or (c) control mechanism that enables liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

Brief Description of the Drawings

Figure 1 A is a top view of an embodiment of refrigeration apparatus with which the present invention may be carried out.

Figure IB is a top view of another embodiment of refrigeration apparatus with which the present invention may be carried out.

Figure 1C is a top view of another embodiment of refrigeration apparatus with which the present invention may be carried out.

Figure 2 is a flowsheet of a mechanical refrigeration circuit useful in the practice of the present invention.

Figure 3 is a plan view of an embodiment of a supplemental heat exchanger useful in the practice of the present invention.

Figure 4 is a flowsheet of an embodiment of the invention including capability to defrost. Figure 5 is a flowsheet of another embodiment of the invention including capability to defrost.

Detailed Description of the Invention

The present invention is useful with any refrigeration apparatus that includes an enclosure in which product is cooled or even frozen by being exposed in the enclosure to a gaseous atmosphere that is colder than the product. The refrigeration apparatus may be a freezer, chiller, cooler, blast cell, cold room, storage room, environmental test chamber, or other equivalent equipment, and may be stationary or transportable. It may be of the type in which product is treated batchwise, that is, batches of product are placed in the enclosure, the enclosure is closed, a desired temperature in the enclosure is maintained, and the temperature of the product is reduced to a desired level by exposure to the atmosphere, following which the enclosure is opened and the product is removed. Refrigeration apparatus with which the invention may be practiced also includes equipment in which product passes through the enclosure on a movable belt or equivalent carrier, wherein the product is placed on a belt at an entrance opening into the enclosure, the belt moves and carries the product through the enclosure wherein the product is cooled or frozen, and the product is removed from the belt at an exit opening of the enclosure. Examples of such apparatus include tunnel coolers and freezers, in which the belt passes on a predetermined path through the enclosure, and so-called spiral coolers and freezers in which the belt moves in a helical path around a central axis such that the belt curves and passes over itself in successive ranks.

The products that can be treated by the present invention include any product that can be cooled or frozen by exposure to the temperatures that are typically established within the refrigeration apparatus. Preferred products include food products, including raw products such as meat (including beef, pork, poultry and fish), and including processed food products comprising combinations of raw materials some or all of which may already have been cooked or otherwise treated.

Referring to Figures 1 A, IB, and 1C, the refrigeration apparatus is designated as 1 and is seen in a top view. Refrigeration apparatus 1 comprises sides 9 which, together with a floor and a top surface that are not seen in this top view, form enclosure 2. Opening 3 through a side 9 can be a doorway (preferably openable and closeable) that is large enough to permit a person to pass into and out of enclosure 2, or opening 3 can be an opening at which an operator outside refrigeration apparatus 1 may place product to be cooled or frozen onto one end of a belt that passes through enclosure 2. Another opening 4 through a side 9 may be provided from which product can be removed from the other end of a belt; or opening 4 can be a second doorway. The atmosphere within enclosure 2 is gaseous and is preferably air. However, the atmosphere within enclosure 2 can instead be air to which has been added one or more other gaseous components.

Refrigeration apparatus 1 also includes mechanical refrigeration unit 5 which comprises mechanical refrigeration circuit 20 that is illustrated in Figure 2. As seen in Figure 2, circuit 20 includes compressor 22, condenser 23, expansion device 24 (which can be a needle valve or equivalent pressure-reduction component), and evaporator 25. These components are connected in a circuit by conduits 26 through which flows a refrigerant such as ammonia or a “Freon” or other suitable halohydrocarbon or hydrocarbon. The refrigerant exiting the evaporator passes to compressor 22, where the refrigerant is compressed, and then to condenser 23 where the refrigerant is cooled, and then through expansion device 24 and then into evaporator 25 wherein refrigerant is vaporized by heat exchange with atmosphere in the enclosure 2. The circuit 20 includes a heat exchange surface 27, which is preferably in evaporator 25; typically the heat exchange surface 27 comprises a coil formed by coiling upon itself the tube which comprises conduit 26 as known by those skilled in the art. In general, evaporator 25 is inside enclosure 2 whereas compressor 22, condenser 23, and expansion device 24 are outside enclosure 2 but one or more of them may be inside enclosure 2.

Returning to Figures 1 A, IB, and 1C, one or more fans 6 or equivalent impellers are preferably provided to move the atmosphere within the enclosure 2 past the heat exchange surface 27 of mechanical refrigeration unit 5 and also to move the atmosphere in heat exchange contact with supplemental heat exchanger 30 as described herein.

Referring to Figure 3, supplemental heat exchanger 30 typically comprises a tube 33 having at one of its ends an inlet opening 34 and at the other of its ends an outlet opening 35. Fins 36 are attached to, and extend radially away from, tube 33, to aid in heat transfer. Tube 33 and fins 36 should be made of metal or another material that conducts heat through itself. Returning to Figures 1 A, IB, and 1C, in the modification of the mechanical refrigeration apparatus in accordance with the present invention, one or more than one supplemental heat exchangers 30 are provided. Each supplemental heat exchanger 30 has an inlet opening 34 which is connected via insulated feed line 32 to a source 31 of liquid nitrogen. Source 31 can be a tank, a truck trailer, or any other piece of equipment that can hold and controllably dispense liquid nitrogen. The flows of liquid nitrogen from source 31 to each supplemental heat exchanger 30 are controlled by appropriate valves and circuitry comprising control mechanism 31 A, whose operation is described herein. Each supplemental heat exchanger 30 also has an outlet opening 35 which is connected to outlet line 39 through which gaseous nitrogen, formed by vaporization of liquid nitrogen in supplemental heat exchanger 30, can pass to vent pipe 37 to be vented to the external atmosphere outside of refrigeration apparatus 1.

As shown in Figure 1 A. supplemental heat exchanger(s) 30 can be positioned within enclosure 2. In this embodiment, line 32 extends from source 31 through an opening in a side 9 to reach inlet opening 34 within enclosure 2. When supplemental heat exchanger 30 is inside enclosure 2, at least one fan 38 may optionally also be present in enclosure 2 to impel atmosphere in heat exchange contact with the tube 33 and fins 36, depending on the mode of operation that is being employed as described herein.

As shown in Figure IB, supplemental heat exchanger(s) 30 can be positioned outside enclosure 2 but attached to the exterior of a side 9. In this embodiment, at each location where a supplemental heat exchanger is attached to a side 9, suitable openings are provided in the side 9 to enable atmosphere within the enclosure 2 to flow out of enclosure 2 into supplemental heat exchanger 30 to be able to have heat exchange contact of the atmosphere with the supplemental heat exchanger 30, and also to enable the atmosphere to flow from supplemental heat exchanger 30 back into the interior of enclosure 2, as illustrated by the arrows B’-B” in Figure IB. When the supplemental heat exchanger 30 is outside enclosure 2, there is preferably at least one fan 38 to help impel atmosphere in heat exchange contact with the tube 33 and fins 36. In this embodiment, gaseous nitrogen can be fed into line 39 and vented to the external atmosphere from vent pipe 37 without having been present inside enclosure 2.

As shown in Figure 1C, supplemental heat exchanger(s) 30 can be positioned outside enclosure 2 and not in contact with refrigeration apparatus 1. In this embodiment, atmosphere is flowed from within enclosure 2 through line 39A into supplemental heat exchanger 30, and from supplemental heat exchanger 30 through line 39B into enclosure 2. One or more fans 38 impel flow of atmosphere from enclosure 2 into supplemental heat exchanger 30, through it, and out of supplemental heat exchanger 30 back to enclosure 2.

The supplemental heat exchanger 30 and its associated lines (such as 32; 37; 39; and 39A and 39B if necessary) can be installed easily into an existing refrigeration apparatus 1 having enough volume within the enclosure 2. If existing openings such as 3 and 4 are not large enough, one of them may be enlarged. Alternatively, as discussed below, it may be advantageous for reasons of heat exchange efficiency and economy to provide more than one supplemental heat exchanger 30 as a plurality of modules, each small enough to fit through an opening such as 3 or 4 into the enclosure 2, wherein the overall heat exchange capacity that can be provided by all of the modules is the sum of what is available from each of the supplemental heat exchangers 30 provided in this manner. A less preferable alternative is to disassemble or cut into pieces a single larger supplemental heat exchanger, pass the pieces into enclosure 2, and reassemble it inside enclosure 2.

Thus, the overall sequence is to install one or more supplemental heat exchangers 30, to provide a line 32 to each supplemental heat exchanger 30 from a source 31 of liquid nitrogen, to provide a connection from each heat exchanger 30 to outlet 35 to line 39 and which is connected to vent pipe 37 for gaseous nitrogen, and to provide controls 31 A for the flows of liquid nitrogen as described hereinbelow.

It should be noted that in one alternative embodiment of the invention, the supplemental heat exchanger 30 is not used to provide heat exchange with the refrigerant of the mechanical refrigeration circuit, and the mechanical refrigeration circuit is not used to provide heat exchange with the liquid nitrogen in the supplemental heat exchanger (nor with nitrogen vapor formed by vaporization of liquid nitrogen in the supplemental heat exchanger); whereas in other alternative embodiments of the present invention, the supplemental heat exchanger 30 is used to provide heat exchange with the refrigerant of the mechanical refrigeration circuit, and/or the mechanical refrigeration circuit is used to provide heat exchange with the liquid nitrogen in the supplemental heat exchanger or with nitrogen vapor formed by vaporization of liquid nitrogen in the supplemental heat exchanger. of the invention

Adding the supplemental heat exchanger 30 as described herein, to a refrigeration apparatus as described herein, provides numerous advantages.

The present invention is useful in refrigeration operational situations in which the mechanical refrigeration system is running to its full refrigeration capacity and capability, or is running at a level of refrigeration that is close to its full refrigeration capacity, e.g. at least 80% or at least 90% of its full refrigeration capacity. In these operational situations, the overall refrigeration capacity that is available is supplemented by the supplemental heat exchanger described herein, that becomes active only when the heat load or operating limitations such as heat rejection outside or frosting inside the enclosure 2 being cooled requires a boost in refrigeration capacity (i.e., additional refrigeration capacity or supplemental refrigeration capacity). The nominal full refrigeration capacity of a mechanical refrigeration unit (unaided by supplemental heat exchange) is readily determined from the unit’s size and refrigerant.

Common issues experienced by processors using mechanical refrigeration systems include: trying to chill products at a production rate that exceeds the capacity of the mechanical system; not reaching the desired product temperature at the end of the cycle of a batch operation or at the exit of a continuous spiral or tunnel freezer; the mechanical refrigeration system capacity diminishes with processing time or seasonally, and thereby effectively reducing available refrigeration capacity provided to chill the product due to e.g. uncontrolled frosting inside the freezer or heat rejection outside the plant due to e.g. climate conditions; or the initial heat load inside the space exceeds the capacity of the mechanical refrigeration system leading to a longer than desired chilling period to mention a few.

Using a supplemental refrigeration system heat exchanger 30 with liquid nitrogen as the refrigerant allows a processor to chill a product heat load that is not capable of being fully cooled or frozen to the desired specifications by the mechanical refrigeration system in the desired time or method when needed. If a processor has no need for additional refrigeration capacity, the supplemental refrigeration system can be deactivated, consuming neither nitrogen nor other significant utilities. The very low refrigerant temperature of liquid nitrogen provides much higher refrigeration capacity per unit volume of the heat exchanger 30 unit than is possible in refrigeration units employing refrigerants other than liquid nitrogen. The difference can be as much as 5-10X more capacity per unit volume of heat exchanger. Even with a design air temperature decrease of as much as 20-30F (compared to 5-15F for conventional refrigerants), the average temperature difference between the air and the liquid nitrogen-based heat exchanger can be as high as 100F to 200F. Therefore, the nitrogen cooled heat exchanger can be easily designed to achieve high temperature changes for the circulated air flow in enclosure 2. This allows the nitrogen cooled heat exchanger to provide substantial amounts of supplemental cooling.

The present invention includes providing the ability to use, and using, liquid nitrogen, which preferably flows through a heat exchanger such as a finned-tube heat exchanger cooled by liquid nitrogen and by vaporized liquid nitrogen inside the tubes to cool the atmosphere in the enclosure, thereby providing additional refrigeration to supplement that provided by the mechanical refrigeration system. When supplemental refrigeration is needed, liquid nitrogen flows into the tubes of the fin-tube heat exchanger, and atmosphere of the enclosure flows across the exterior surfaces of the supplemental heat exchanger. The liquid nitrogen vaporizes, absorbing heat from the gas flow across the exterior surface (e.g. fins and tubes) of the supplemental heat exchanger, and converting some or all of the liquid nitrogen into gaseous nitrogen. The warmed but still cool gaseous nitrogen can provide additional cooling in supplemental heat exchanger 30. Eventually, this gaseous nitrogen leaves the exit of the supplemental heat exchanger and is safely vented via line 39 and vent pipe 37 to the atmosphere outside of the facility. The cooled air flow is directed within the enclosure 2 to provide a refrigerated air flow as needed for the purpose of the enclosure. The nitrogen-cooled supplemental heat exchanger 30 operates in concert with, but separately from, the mechanical refrigeration unit 5, to provide supplemental cooling only when needed.

Direct feeding of liquid nitrogen refrigerant into the supplemental heat exchanger’s coil tubing can result in two zones of heat transfer - a latent zone where both liquid and vapor flow of nitrogen are present, followed by a superheat zone where only nitrogen gas is present. The latent zone operates at temperatures near the saturation temperature of nitrogen at coil conditions (nominally -3 OOF), while the superheat zone starts at about saturation temperature and the gas heats up as it proceeds down the tube, exiting the tube (coil) outlet at the design gas exit temperature (in the range of -80F to -50F, preferably 20F to 50F colder than the supply air temperature on to the coil. Colder temperatures for the exiting gas are possible but attainment may be at the expense of the cooling efficiency of a given mass quantity of the nitrogen.

Unlike conventional operations with refrigerants, the nitrogen cooled heat exchanger coil obtains substantial refrigeration capacity from the superheat zone. Conventional refrigerants typically employ superheats of only a few degrees F above the saturated suction temperature, and only a small part of the refrigeration load is provided by the sensible warming of the refrigerant in the superheat region. In contrast, about 40% of the refrigeration provided by the supplemental heat exchanger of the present invention fed with liquid nitrogen is provided in the superheat region, with the other 60% of the duty provided from the latent zone.

Where supplemental heat exchanger designs with coils of several tube rows are employed, it is preferred to use a single nitrogen circuit, with the nitrogen flowing countercurrent to the direction of air flow. The single circuit and multiple tube rows serve to spread out the temperature variation of the superheat region over the entire air flow, promoting even cooling, and the counter-current flow of air and refrigerant increases the efficiency of the heat exchange. However, the load capacity of the nitrogen cooled coil is not substantially reduced if other flow patterns are used - for example, multiple circuits, co-current flow, or a single tube row coil.

The present invention can be based on either of the following two approaches to implementing the addition of supplemental cooling to the refrigerated enclosure 2. The approach chosen for a specific instance will depend on the needs and constraints of the existing refrigeration system and facility.

• Common air flow: nitrogen-cooled heat exchanger coils are positioned in the existing air flow of the refrigerated space, to provide additional cooling without their own dedicated mode of air movement.

• Independent air flow: nitrogen-cooled heat exchanger coils are provided, along with their own dedicated means of air movement (such as fans or blowers), to provide additional cooling to the air in the refrigerated space. Use of nitrogen-cooled coils for supplemental refrigeration - common air flow

The general practice by manufacturers of freezers using a mechanical refrigeration system is to place the coil system of the mechanical system within the freezer enclosure. The mechanical refrigeration system may contain one large coil by design, but often the mechanical refrigeration system can have multiple coils acting as one system to provide cooling capacity. A heat exchanger system providing supplemental cooling capacity using liquid nitrogen can also be located within the enclosure 2 as shown in Figure 1 A. In Figure 1 A, a single supplemental heat exchanger 30 is shown inside the enclosure 2 and provides additional cooling capacity when the heat load inside the enclosure approaches, equals, or exceeds the capacity of the mechanical refrigeration system capacity. The supplemental nitrogen-cooled coil may be placed in close proximity to the mechanical refrigerant coils, sharing the same airflow.

The preferred embodiment of this invention for a shared or common air flow is as follows:

• Nitrogen coils that are shallow (1-3 tube rows) with wide fin spacing (0-6 fins per inch, preferably 0-3 fins/inch), and in-line tube arrangement. These features, combined with the very low temperatures provided by the liquid nitrogen refrigerant, enable substantial supplemental capacity to be delivered to the refrigerated space without introducing excessive pressure drop to the common air flow. The ratio of the air side pressure drop of the nitrogen coil to that of the existing mechanical coils should be <0.15, most preferably < 0.10, so as not to excessively reduce the air flow and refrigeration efficiency of the combined system.

• For nitrogen coils placed downstream and in close proximity to the existing mechanical coils, “close proximity” means that the gap between the mechanical coil and the nitrogen coil (from tube centerline to tube centerline) is as small as achievable for the constraints of the installation. It is preferred that the gap is <12D, preferably 3.6D to 12D, more preferably 2.4D to 8D, and most preferably 1.2D to 4D, where D is the diameter of the nitrogen coil tubing in the supplemental heat exchanger. Keeping this gap small promotes streamline flow and minimizes abnormal air velocity vectors and recirculation, to promote good heat transfer efficiency. Positioning the nitrogen coil as close as practical to the face of the mechanical coil also reduces the intrusion of the added coil on the interior space of the refrigerated enclosure, to minimize impact on maintenance or other required activities, an important factor in retrofit of a supplemental system.

• Nitrogen coils of the supplemental heat exchanger 30 are preferably installed in small, modular pieces to allow easy installation through small hatchways and inside existing equipment with limited space by 1-2 people without heavy lifting equipment. Each module would have short dimensions <36” and longest dimensions <60”, and weigh less than 150 lbs.

• The modular nitrogen coil subassemblies (coil sections, supports, headers, defrost pans, isolation louvers) are assembled inside the refrigerated space to create the desired operating configuration of the nitrogen supplemental system. Coil subassemblies include various features (brackets, tabs, slots, hangers, connecting means) to facilitate interconnection within the space.

For example, in the case of one system, a 1-row deep liquid nitrogen coil added to a mechanical refrigeration unit having a 10-row coil containing ammonia as the refrigerant, configured to provide full face-area coverage of the existing coil, could provide up to 50% more refrigeration capacity with less than 10% additional air-side pressure drop. In contrast, using methods of the prior art to increase capacity, adding a second 10-row coil cooled by ammonia, downstream of the existing 10-row coil, would add only 30% to the cooling capacity, and would double the air-side pressure drop. The supplemental nitrogen-cooled coil unexpectedly provides about 17 times the refrigeration capacity per coil row, as compared with extending the number of rows of the mechanically cooled coil.

Use of nitrogen-cooled coils for supplemental refrigeration - independent air flow

Alternatively, the supplemental nitrogen-cooled coils may be placed within the freezer enclosure but at a distance from the mechanical refrigerant coils, and may have their own independent air circulation fan or multiple fans or blowers. Freezer designs often do not have space available to allow the installation of one supplemental coil and becomes even more challenging to accommodate more than one supplemental coil using nitrogen as the refrigerant. To address the space limitations inside a freezer with a mechanical refrigeration system, this invention describes attaching compact, modular nitrogen coil units to a wall or ceiling of a freezer and using a couple of holes to direct air to and from the supplemental coil system as shown in Figure IB. This approach is possible as the fin-tube coil designs using nitrogen have a much smaller size and volume in comparison to a similar refrigeration capacity coil using either ammonia or freon refrigerants. The compact size allows one or multiple coils to be installed on a freezer to provide the necessary supplemental refrigeration capacity desired. A third approach is to provide supplemental refrigeration capacity using nitrogen as the refrigerant with one or more coils assembled in a secondary enclosure separate from the freezer boundary and using ducts for incoming and outgoing fluid transfers with the internal freezer volume of air, as shown in Figure 1C.

The preferred embodiment of this invention for an independent air flow configuration is as follows: o For the independent air flow embodiment, a multi-row coil (3 to 10 rows, more preferably 4 to 7 rows) is preferred, with a compact face area (<48” in width and <36” in height of the face) is preferred, to better accommodate the small, modular unit concept. The preferred fin arrangement is a variable fin count, with wider spaced fins on the superheat region (in the initial zone of air flow) to allow frost capture without clogging of the coil, followed by narrower fin spacing in the latent zone of the coil, where the load per unit length of the tubing is higher.

• Supplementary heat exchange with liquid nitrogen is provided in the form of one or more modular cooling units (MCU). Each MCU will be made from one or more subunits, comprising at least one nitrogen-cooled heat exchanger or coil subunit, and may also include an air-supply subunit (which may include one or more fans or blowers), an airreturn subunit, and a control subunit. Preferably, the MCU will comprise an assembly of at least one each of air supply, coil, air return, and control subunits.

• Each subunit (air supply, coil, air return, and control) are small, modular pieces to allow easy installation through small hatchways and inside existing equipment with limited space by 1-2 people without heavy lifting equipment. Each of the subunits, in turn, may comprise smaller component assemblies, as is needed to limit the size and weight of materials to be handled during installation. For example, the air supply subunit may be configured of three component assemblies, one being the enclosure assembly, combined with two fan component assemblies.

• Subunits and component assemblies will have short dimensions <36” and longest dimensions <60”, and will weigh less than 150 lbs. each.

• Each MCU, comprising the combination of appropriate subunits and component assemblies, will be assembled inside the refrigerated space, or on the walls, ceiling, or adjacent to the refrigerated space, to create the desired operating configuration of the nitrogen supplemental system. MCU subunits and component assemblies include various features (brackets, tabs, slots, hangers, connecting means, electrical and tubing plugs and quick-connects) to facilitate interconnection within the space.

• MCU subunits are designed to allow installation in various configurations, using the same basic subunit design with or selecting from one or more variations of similar subunit designs. o Vertically on side wall of refrigerated space - either outside of wall, inside of wall, or through wall. o Horizontally on side wall of refrigerated space, o On ceiling or below floor of refrigerated space.

• The following is an example of the installation of one MCU in vertical orientation on outside wall of a refrigerated space, to illustrate how small, compact, easy to handle subunits may be used to facilitate installation in a retrofit environment. First, a prefabricated support frame may be installed on the outside wall. Two holes are cut through the insulated wall of the refrigerated space to allow air supply and return to the MCU, and prefabricated frames installed through the openings. An air supply subunit is attached to the top of the frame and to the air supply frame. Two fan component assemblies are inserted into the air supply subunit and connections made. The coil subunit is attached beneath the air supply subunit, secured to the frame and to the air supply subunit. Coil and insulating panel component assemblies are inserted into the coil subunit. The air return subunit is attached beneath the coil subunit, secured to the support frame and coil subunit, and a defrost pan component assembly is inserted into the air return subunit. A control subunit is attached to the air return subunit, final interconnections are made between subunits, and remaining outside panels are attached to enclose the assembled MCU. Final piping and electrical connections are made to the MCU.

• After determining the overall amount of supplemental refrigeration to be delivered to the refrigerated space, the number of MCUs to be installed is determined. Where defrost of the MCUs will be necessary during operation of the refrigerated space, extra MCU units should be installed to allow some units to be operating while others are in defrost or in stand-by. The compact, modular size and high capacity per unit volume of the MCUs enables the retrofit of the desired capacity into the existing mechanical system within space constraints. This design approach also facilitates the installation quickly and with minimal disruption and expense.

• For example, the installation of one MCU of 30”x60”x22” as assembled can provide over 3 tons of supplemental refrigeration. Surprisingly, two of these units, in total occupying less than 50 cubic feet of space, in operation can provide enough supplemental refrigeration to boost food processing capacity by about 20% for a 5,000 cubic foot spiral freezer system that currently freezes a food load demanding 35 tons of refrigeration. Unexpectedly, the freezer capacity can be increased 20% with the addition of only 1% to the volume of the equipment installation.

• Different designs for each subunit can be employed to provide MCU systems of different capacities of air flow or refrigeration load. For example, an air supply subunit could be fitted with three different types of fan component assemblies for different air flow capacities. Different coil subassemblies can be employed to provide different overall refrigeration loads per unit. Combinations of air flow and coil capacities may be employed to provide different values of air-side temperature drop and air flow rate.

• Sensors and controls may be centralized, with control signals sent to individual MCUs to control their operation according to the current need for supplemental refrigeration. However, preferably, sensors and controls may be integrated into each MCU so that they can operate independently. For example, the air intake fans may be configured to periodically create air flow into the air intake, wherein a temperature senor determines the air temperature of the immediate environment in the refrigerated space. If the air temperature is above setpoint, the control system puts the MCU into active operation. • In some cases, independent control may need to be modified by incorporating communication among MCUs. Communication means may be installed in each MCU to allow information to be passed between them, to take appropriate control actions. For example, one MCU that goes into defrost mode may alert other nearby MCUs of that status, enabling them to increase their output in a feed-forward manner to overcome the local shortfall in refrigeration. Another example is an MCU that is running at full output and is not able to reduce the local temperature within setpoint. In that case, the neighboring MCUs can increase output (above that determined by their own local sensors) to provide additional supplementary cooling. This type of network communication can ease installation, by eliminating the need for integrating and programming a centralized control scheme.

The modular design of the MCU system allows the cooling with supplemental heat exchangers 30 to be distributed in an optimum fashion throughout the extent of the enclosure 2. This can be determined during planning of the installation, after analyzing the spatial variation of heat loads in the space, as well as the operation and air flow of the existing mechanical refrigeration system, to determine the desired placement and distribution of the MCUs in the existing system. One variant of placing supplemental refrigeration coils on freezer walls is to locate one or more coils in the vicinity of the freezer inlet port of a spiral freezer or tunnel freezer. One advantage of using nitrogen as the refrigerant in a fin-tube coil design is that the heat exchange surface of the supplemental heat exchanger, which contacts the atmosphere in enclosure 2, is colder than the heat exchange surface of the mechanical refrigeration circuit. This gives the supplemental heat exchanger the capability of forming a region in the enclosure 2 in which region the atmosphere is colder than the air stream that is cooled only by the mechanical refrigeration system. This capability to create a colder temperature zone inside the enclosure 2 can provide greater tempering of the incoming product needing to be cooled. Particularly, this cold zone can reduce moisture loss off from incoming product. By keeping moisture on the product, the frost that would have formed due to the water leaving the product does not reach the mechanical refrigeration coil and therefore slows the rate of frost buildup on the mechanical refrigeration system coil. Figure IB illustrates an arrangement that takes advantage of this capability, wherein a supplemental heat exchanger 30 is positioned near opening 3 through which product enters into enclosure 2. Relationship of supplemental coils to existing coils and air flow

The supplemental heat exchangers 30 may be placed upstream, downstream, or in parallel to the mechanical refrigerant coils, with respect to the main circulation pattern of air flow within the freezer. The supplemental coils may share a common air flow path with the mechanical coils, or may have a separate air flow path, and may have their own dedicated means of air movement such as fans. When the supplemental coils share the same air flow as the mechanical refrigeration coils, a preferred embodiment is for the supplemental coils to be downstream of the mechanical coils, i.e., the warm air would first be cooled by the mechanical coils, and then further cooled by the supplemental nitrogen-cooled coils.

For the independent airflow embodiment of this invention, the supplemental heat exchangers 30 for supplemental liquid nitrogen-based refrigeration may be installed either downstream or in parallel to the air flow pattern of the existing mechanical refrigeration system. “Downstream” means positioned in the air flow after it is cooled by the mechanical coils, but before the air returns to the mechanical refrigeration apparatus. “In parallel” means that the air flow to the nitrogen cooled supplemental units is obtained upstream (before being cooled) of the mechanical coils. For example, in a spiral freezer, a parallel flow supply would entail warm air returning from the food load to be provided into the air supply of the MCU, and the cooled air leaving the air return of the MCU being directed back to the food load.

The objective of the independent air flow embodiment of the present invention is not that the MCUs will provide an increase in the overall air circulation over the food product or other load within the refrigerated space. Instead, the air flow of the MCUs is meant only to remove air from the space, provide additional cooling, and return that air to the space, while the existing air flow of the mechanical system continues to provide required air circulation to the enclosed loads. The MCUs need only be positioned where additional cooling capacity can be beneficial in the refrigerated space, where the air flow of the MCU will not cause substantial disruption of the base mechanical air flow, and where the cold exit air of the MCUs will not adversely impact the operation of the mechanical refrigerant coils. Since the MCUs can provide substantial (10-30%) increases in cooling capacity with only small independent air flows (5-15%), any potential adverse impacts of the MCU airflow can be easily avoided. If the objective is to lower the air temperature supplied to the heat load to below that normally provided by the mechanical refrigeration system, then positioning the nitrogen cooling system downstream of the mechanical coils is preferred. On the other hand, if the objective is to provide additional cooling capacity at the same air temperature normally achievable by the mechanical refrigeration system, then parallel air flow for the MCUs can provide higher supplemental capacity. Either parallel or downstream air flow patterns will work well to modify the bulk air flow stream in the enclosure 2.

As mentioned above, the very large temperature difference between air and refrigerant in the nitrogen coils allows efficient operation of the system with substantial air temperature drops (20F to 30F), and this allows the system to provide substantial supplemental refrigeration even when the affected air flow is a small fraction of the overall circulation rate. For example, if the existing mechanical system operates with a 10F air temperature drop across the mechanical coils, and a 30F drop across the nitrogen coils, the nitrogen cooled MCUs can provide a boost in refrigeration capacity of 15% to the system while processing only 5% of the system air flow.

Another aspect of this invention is that each coil of the supplemental refrigeration system acts independent of the other coils when more than one coil is active. For a supplemental refrigeration coil, especially one using nitrogen as the refrigerant, allows for different refrigeration capacity levels in the different active coils. For example, if two supplemental coils were active and each having the same overall refrigeration capacity, one coil may provide 100% of the rated capacity while the second coil might be contributing 70% of the rated capacity. In a different scenario, these two coils can be operated in unison with both providing 85% of the rated capacity. Independent flow control of nitrogen through a coil provides the capability to deliver a precise amount of refrigeration cooling capacity to satisfy a demand.

Control of the Integrated system

The supplemental refrigeration heat exchanger 30 using liquid nitrogen as the refrigerant can be turned on or made active, and the flow of liquid nitrogen once activated, can be controlled using any of several different methods. In any method, it is preferred that the mechanical refrigeration system is operating at full capacity before activation of the flow of liquid nitrogen into the supplemental refrigeration heat exchanger using nitrogen. However, the control practices can also be carried out while the mechanical refrigeration system is operating but before the mechanical refrigeration system has reached full capacity, such as when it has reached a preset level such as at least 80%, or at least 90% or at least 95%, (but, as will be apparent, preferably greater than 0%) of the full refrigeration capacity of the mechanical refrigeration system without any supplemental heat exchange.

One method is to monitor the operation of the mechanical refrigeration system, and to activate the flow of liquid nitrogen into the supplemental refrigeration heat exchanger, when the mechanical system is at maximum load or is at a level that is a preset percentage of maximum load. Various signals or sensors may be used to indicate the operating capacity of the mechanical system, including suction pressure, speed, or slide valve level of the mechanical refrigerant compressor, from which the maximum heat exchange capacity can be determined depending on the specific design of the mechanical refrigeration system. When the sensors indicate that the mechanical refrigeration system is operating at full capacity, the flow of liquid nitrogen into the supplemental refrigeration heat exchanger (coils) is activated to provide additional refrigeration capacity to cool the atmosphere in enclosure 2. The supplemental heat exchange system would then continue to operate as long as the mechanical refrigeration system remains at full load, or as long as it remains at a load that is at or above the preset percentage of full load, depending on how the operator chooses to operate the overall system. Once the utilization of the mechanical refrigeration system’s capacity diminishes to less than maximum utilization, or to less than the aforementioned preset percentage of maximum capacity, then the supplemental refrigeration system can be reduced in capacity (such as by reducing the flow rate of liquid nitrogen through the supplemental heat exchanger), or can be turned off fully. The supplemental refrigeration system would provide capacity linked to the monitored capacity parameter of the mechanical system. This method of activating the supplementary heat exchanger requires integration of sensor signals from the mechanical refrigeration system, and may be complicated when the mechanical system serves multiple freezer loads.

Another method, which is preferred, of controlling actuation and operation of the supplemental refrigeration heat exchanger is to rely on one or more characteristics of the mechanical refrigeration system which are correlated with the refrigeration performance of the mechanical refrigeration system, to indirectly determine when conditions have been established which indicate that the supplemental heat exchanger is to be activated by starting the flow of liquid nitrogen into the heat exchanger, as well as the conditions which indicate when the flow of liquid nitrogen is to be lessened or shut off.

One embodiment of such a method is to use a combination of monitoring the temperature of the atmosphere within enclosure 2 and monitoring the pressure difference of the liquid nitrogen flow through the tube of the supplemental heat exchanger. For a given mechanical refrigeration system, the suction pressure and resulting air temperature inside the enclosure 2 will be known. As the temperature inside the enclosure 2 warms and the utilization of the refrigeration capacity of the mechanical refrigeration system approaches or reaches full capacity, the mechanical refrigeration system will become less able and eventually unable to chill the product to the extent desired by the operator. As a result, the temperature inside the enclosure 2 rises. When that temperature has risen sufficiently to and reaches a set point that has been set by the operator, which corresponds to a given percentage of the maximum refrigeration capacity of the mechanical system having been reached, the flow of liquid nitrogen into the supplemental heat exchanger system is turned on by opening of appropriate valves between source 31 and inlet 34, and supplemental heat exchange capacity is provided to the system.

The amount of supplemental refrigeration capacity provided will continue to be provided as long as the temperature inside the enclosure 2 remains warmer than the temperature that the atmosphere would reach when the mechanical system again has sufficient capacity to maintain the desired temperature without the aid of the supplemental refrigeration heat exchanger. When the temperature of the atmosphere in the enclosure 2 begins to cool and returns closer to the target temperature of the atmosphere in enclosure 2, the amount of refrigeration capacity provided by the supplemental refrigeration system is reduced, such as by reducing the flow rate of liquid nitrogen into the supplemental refrigeration heat exchanger. When the temperature in enclosure 2 returns to the desired set point temperature, the supplemental refrigeration system is turned back or shut off into an idle mode until the refrigeration demands of the mechanical refrigeration system again overwhelms the mechanical refrigeration cooling capacity.

When the supplemental refrigeration heat exchanger system is active, control of the liquid nitrogen flow into it is controlled by maintaining the pressure difference predicted by the current temperature difference between the desired temperature of the atmosphere in enclosure 2, and the actual temperature of the atmosphere. As this temperature difference becomes larger than desired, the predicted pressure difference for the supplemental heat exchanger increases and the flow of liquid nitrogen into the heat exchanger is increased. Similarly, if this temperature difference becomes smaller, the pressure difference target is reduced, and this action reduces the nitrogen flow rate into the supplemental heat exchanger. For some installations, the temperature difference between the desired atmosphere temperature in the enclosure 2 and the actual temperature may include an offset to prevent overlapping control zones for nitrogen flow. When the temperature difference becomes negligible (meaning the desired temperature of the atmosphere in enclosure 2 and the actual temperature there including any offset are essentially equal), the supplemental refrigeration heat exchanger system is put into a standby condition. In this standby condition, no nitrogen flows into the heat exchanger and the coil sits idle not providing any additional refrigeration capacity. Other characteristics that may be detected and used to control operation of the system include measuring the temperature or thermal state of the product as it exits the enclosure 2; a temperature or thermal state that is above a preset set point temperature or thermal state of the product would cause the system to be activated to provide additional supplemental refrigeration that would lower the temperature or thermal state of the product as it exits the apparatus.

Nitrogen flow in aid of operations can be measured in other techniques as well, such as by use of a flow meter or equivalent apparatus and methodology.

The flow of nitrogen refrigerant into the supplemental heat exchanger can be controlled in any of a variety of ways, including expanding or reducing the flow of liquid nitrogen into the heat exchanger, or expanding or reducing the flow of nitrogen vapor leaving the heat exchanger, using suitable valves. Alternatively, the flow of nitrogen may be controlled in an on-off fashion using suitable solenoid-controlled valves either upstream or downstream of the supplemental heat exchanger. In the preferred embodiment, the flow is controlled using a control valve on the nitrogen gas flow leaving the supplemental heat exchanger (to line 39). It is further preferred to control the flow in a continuous fashion, over a range of flow rates, in concert with the control strategy described herein, to vary the supplemental refrigeration load as needed. Defrosting

Refrigeration apparatus that relies on heat exchange between a cold surface of a heat exchanger and the surrounding atmosphere, such as mechanical refrigeration systems employing either ammonia or freon as the refrigerant, and other systems such as the supplemental heat exchanger described here that employs liquid nitrogen, generate frost on the heat exchange surfaces which is formed by condensation of water vapor from the atmosphere and freezing of that condensed water on the heat exchange surfaces. The present invention includes an advantageous approach to removing frost from heat exchange surfaces on which the frost has formed.

In this aspect of the present invention, referring to Figure 4, recycle line 41 through which gaseous nitrogen can flow is established between its inlet 41 A and its outlet 41B. Inlet 41 A is located in line 39 between outlet 35 and vent 37, and outlet 41B is located in line 32 between source 31 and inlet 34. Flow of nitrogen into recycle line 41 or to vent 37 is controlled (that is, whether flow is permitted or is shut off) by valve 42 in line 42 and by valve 44 in line 39. Valve 45 in line 32 and valve 43 in line 41 control whether nitrogen that enters inlet 34 of supplemental heat exchanger 30 is the liquid nitrogen from line 32 or is gaseous nitrogen from line 41. An impeller 46, which can be a blower or a compressor or a fan or other equivalent equipment, is provided in line 41, to circulate the gas in recycle line 41. Additionally, a heating element 47 is provided in line 41 to heat gas that is flowing through line 41.

The impeller and heater are designed to allow adequate turbulent flow of hot gas within the circuit that includes recycle line 41 to deliver sufficient heat to fully defrost the supplemental heat exchanger in the time desired. For example, nitrogen from the impeller 46 may be heated to 200F and supplied to the inside of the recycle line 41 at a high velocity in turbulent flow, to heat the supplemental heat exchanger from within, melting the ice within approximately 30 minutes. Hot gas may be first routed via line 51 through a defrost pan 52 situated beneath the supplemental heat exchanger, to heat the pan 52 and prevent liquid water that is formed by melting of the frost and that drains off of the supplemental heat exchange from re-freezing in the pan 52, so that it can be drained as desired during or after the defrost process. 1 In the operation described previously of using the supplemental heat exchanger to supplement the refrigeration capacity of a mechanical refrigeration system, valves 45 and 44 are open, and valves 43 and 42 are closed.

Defrosting of the supplemental heat exchanger 30 (that is, melting of ice from the surfaces of the heat exchanger) involves establishing a closed loop of warmed, circulating nitrogen vapor. This is established by closing valves 45 and 44 and opening valves 42 and 43 and passing nitrogen vapor under the influence of impeller 46 through heater 47, into line 32 and through supplemental heat exchanger 30, into line 39 and then into line 41. More preferably, air continues to circulate over or across the supplemental heat exchanger, providing heat to remove remaining nitrogen liquid in the heat exchanger; during this process step it continues to cool the air. Of course, one can isolate the supplemental coil immediately to allow defrost to commence in an isolated fashion, but this approach is less preferred. The next step is to reduce the pressure level in the recycle line 41 to a level below the normal operating pressure of the nitrogen circuit including the supplemental heat exchanger, to a value below the maximum operating pressure of the impeller being employed. If not already activated, heater 47 is activated, preferably to a temperature on the order of 120F to 21 OF, preferably 160F to 200F, as detected by temperature sensor T3 in Figure 4.

The nitrogen vapor that is defrosting the coil is passed through this recycle loop more than once, preferably many more times than once. The hot gas circulation continues until the ice melts from the surface(s) of the supplemental heat exchanger is defrosted (preferably, until all ice present is fully melted). The extent of melting can be determined by monitoring the temperature of the gas downstream of the supplemental heat exchanger (such as temperature sensors T1 in Figures 4 and 5 that measure temperature of the gas in each line 39), before the gas re-enters impeller 46. Initially this gas will be cold but it will warm as the defrost operation continues. When all of the ice is melted, the load on the defrost circuit changes significantly, such that the temperature of the gas in the line at T1 will rise at a more rapid rate. A sensor is preferably provided in the control system that can detect this change in the temperature, or the change in the rate at which the temperature is rising, and terminate the defrost cycle in response to the detected change. Alternatively, one or more temperature sensors T2 (seen in Figure 4) can be provided at a surface of the supplemental heat exchanger to detect when the temperature at that surface exceeds 32F signifying the absence of an ice layer. As another alternative, capacitance probes designed to detect the presence of ice may be used in place of (or in addition to) the temperature sensor(s) T2.

The time is typically about 30 minutes or so for a supplemental heat exchanger 30 having refrigeration capacity of 5 tons of refrigeration.

To improve the defrost time, a preferred method is to isolate the supplemental heat exchanger, using baffles or other means known in the art, after the liquid nitrogen has boiled out of the heat exchanger. By isolating the heat exchanger, a supplemental heat exchanger can be defrosted asynchronously to the time when the mechanical refrigeration system is defrosted and reduces the parasitic heat load back into the mechanical freezer interior volume. Since the defrost step for a supplemental heat exchanger coil by design can be any time when needed, a supplemental heat exchanger can be defrosted at the same time that the mechanical refrigeration system defrost is occurring if that makes the defrosting of the supplemental heat exchanger more convenient for an operator.

Referring to Figure 5, with appropriate valving one or more supplemental heat exchangers 30 using liquid nitrogen as the refrigerant can be defrosted at differing times or at the same time. As shown in Figure 5, a single common impeller 46 and a single heater 47 can be used to provide defrosting to two different supplemental heat exchangers 30 in the same refrigeration apparatus. The heat exchangers may be defrosted simultaneously, or in sequence. The operation is as described with reference to Figure 4, except that valves 42, 43, 44, and 45 are opened and closed at points in time that create the desired recycle loops for defrosting each supplemental heat exchanger 30. As with the system shown in Figure 4, the nitrogen vapor that is used to defrost the coils is passed through the recycle loops more than once, preferably many more times than once.

When more than one supplemental heat exchanger is active at the same time, and to allow for an additional supplemental heat exchanger to asynchronously go into defrost mode relative to the mechanical refrigeration system defrost mode, a pattern of activity can be established so each of the supplemental heat exchangers is active for a period of time, then proceeds through a defrost sequence, and then becomes active once again or is put into a standby mode depending on freezer conditions. For example, a supplemental refrigeration system can be designed for each supplemental heat exchanger to be in operation for 90 minutes (three contiguous thirty- minute periods), followed by one 30-minute defrost period. Other patterns and segment lengths may be utilized, depending on the capacity of the system, the system design, and the needs of the process.

The present invention is more efficient than systems employed in the prior art as no additional nitrogen is needed from storage to implement the defrost operation.

Other techniques for providing the desired defrosting, to melt ice from a surface of a heat exchanger, may be utilized or incorporated as well with apparatus and/or methodology comprising any of the embodiments or features described herein. Examples of suitable techniques include directing a stream of hot gas onto the ice; incorporating electrical heating elements such as heating wires or heating rods, interspersed among tube rows; spraying liquid, such as warm water, onto the ice surface; blowing ambient air across the ice surface; periodically discontinuing flow of refrigerant through the heat exchanger, enabling the ambient conditions to cause the ice to melt; and incorporating tubes interspersed among the refrigeration coils, and flowing heated liquid such as heated brine or glycol through the incorporated tubes.

Claims

WHAT IS CLAIMED IS:

1. A method of modifying refrigeration apparatus, comprising providing refrigeration apparatus to be modified that comprises an enclosure enclosing an atmosphere, and that comprises a mechanical refrigeration circuit comprising within the enclosure an evaporator having a heat exchange surface in contact within the enclosure with the atmosphere, the mechanical refrigeration circuit also comprising a compressor, a condenser, and an expansion device, which are connected together with the evaporator in fluid communication so that circulation of a refrigerant through the circuit cools the heat exchange surface and can thereby cool the atmosphere within said enclosure; the refrigeration apparatus also including a refrigeration impeller that circulates atmosphere into contact with said heat exchange surface; and modifying the refrigeration apparatus by adding a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which can be coupled to a vent that is open to the atmosphere outside the enclosure, and positioning the supplemental heat exchanger so that atmosphere that has been cooled by contact with the heat exchange surface of the mechanical refrigeration circuit in the enclosure can be flowed in heat exchange contact with the supplemental heat exchanger, and adding control mechanism that enables liquid nitrogen to flow from said source into the supplemental heat exchanger to further cool, by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, said atmosphere that has been cooled by heat exchange with said mechanical refrigeration circuit, and that enables said liquid nitrogen to flow into the supplemental heat exchanger only when the cooling of the atmosphere in the enclosure that is provided by said mechanical refrigeration circuit without said further cooling is at the maximum that can be provided by said circuit or is within a preset percentage below said maximum.

2. A method according to claim 1, further comprising modifying the refrigeration apparatus by adding a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, and adding control mechanism that enables the flow of nitrogen to be alternated between a first mode in which nitrogen can flow from the outlet of the supplemental heat exchanger into said recycle line and not to said vent and nitrogen from said recycle line that has been heated by said heater and not liquid nitrogen from said source thereof can flow into the inlet of said supplemental heat exchanger and the nitrogen from said recycle line can be recycled into said supplemental heat exchanger repeatedly, and a second mode in which nitrogen can flow from the outlet of said supplemental heat exchanger to said vent and not into said recycle line, and liquid nitrogen from said source and not nitrogen from said recycle line can flow into the inlet of said supplemental heat exchanger.

3. A method according to claim 1 wherein there is no heat exchange between nitrogen in the supplemental heat exchanger and the refrigerant of the mechanical refrigeration circuit.

4. A method according to claim 1 wherein the control mechanism that is added to enable liquid nitrogen to flow into the supplemental heat exchanger enables said flow only when the cooling of the atmosphere in the enclosure that is provided by said mechanical refrigeration circuit without said further cooling of the atmosphere is at the maximum that can be provided by said circuit or is within a preset 90% of that maximum.

5. A method according to claim 4 wherein said control mechanism enables said flow when it senses that the temperature of the atmosphere in the enclosure is higher than a temperature that corresponds to the cooling provided by said circuit being within 90% of said maximum.

6. A method according to claim 1 wherein the control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

7. A method according to claim 1 comprising adding (a) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source, or (b) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger, or (c) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

8. A method according to claim 1 wherein the supplemental heat exchanger is added inside the enclosure.

9. A method according to claim 1 wherein the supplemental heat exchanger is added outside the enclosure.

10. A method according to claim 9 wherein the supplemental heat exchanger physically shares a wall of the enclosure.

11. A method according to claim 1 wherein all circulation of the atmosphere within the enclosure is provided by said refrigeration impeller.

12. A method according to claim 1 further comprising adding a supplemental impeller in said enclosure and wherein circulation of said atmosphere within the enclosure is provided by said refrigeration impeller and by said supplemental impeller.

13. A method according to claim 1 further comprising adding controllable mechanism that can melt ice that has formed on a surface of the supplemental heat exchanger.

14. A method according to claim 1 wherein the supplemental heat exchanger is capable of cooling atmosphere in a portion of the enclosure to a temperature that is lower than the temperature to which the mechanical refrigeration circuit cools the atmosphere.

15. A method of modifying refrigeration apparatus, comprising providing refrigeration apparatus to be modified that comprises an enclosure enclosing an atmosphere, and that comprises a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which can be coupled to a vent that is open to the atmosphere outside the enclosure, wherein the refrigeration apparatus comprises control mechanism that controllably enables liquid nitrogen to flow from said source into the supplemental heat exchanger to cool said atmosphere by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, and that controls the rate of flow of said liquid nitrogen into the supplemental heat exchanger; modifying the refrigeration apparatus by adding a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, and adding control mechanism that enables the flow of nitrogen to be alternated between a first mode in which nitrogen can flow from the outlet of the supplemental heat exchanger into said recycle line and not to said vent and nitrogen from said recycle line that has been heated by said heater and not liquid nitrogen from said source thereof can flow into the inlet of said supplemental heat exchanger and the nitrogen from said recycle line can be recycled into said supplemental heat exchanger repeatedly, and a second mode in which nitrogen can flow from the outlet of said supplemental heat exchanger to said vent and not into said recycle line, and liquid nitrogen from said source and not nitrogen from said recycle line can flow into the inlet of said supplemental heat exchanger.

16. A method according to claim 15 wherein the control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

17. A method according to claim 15 comprising adding (a) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source, or (b) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger, or (c) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

18. A method according to claim 15 wherein the supplemental heat exchanger is inside the enclosure.

19. A method according to claim 15 wherein the supplemental heat exchanger is outside the enclosure.

20. A method according to claim 15 wherein the supplemental heat exchanger physically shares a wall of the enclosure.

21. A method of refrigerating a product, comprising providing the product in an enclosure that contains an atmosphere, wherein a refrigeration apparatus is present that comprises a mechanical refrigeration circuit comprising within the enclosure an evaporator having a heat exchange surface in contact within the interior space with the atmosphere, the mechanical refrigeration circuit also comprising a compressor, a condenser, and an expansion device, which are connected together with the evaporator in fluid communication so that circulation of a refrigerant through the circuit cools the heat exchange surface and can thereby cool the atmosphere within said enclosure; the refrigeration apparatus also including a refrigeration impeller that circulates atmosphere into contact with said heat exchange surface; and wherein a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which is open to the atmosphere outside the refrigeration apparatus is positioned so that atmosphere that has been cooled by contact with the heat exchange surface of the mechanical refrigeration circuit can be flowed in heat exchange contact with the supplemental heat exchanger, and cooling atmosphere in the enclosure by heat exchange with the heat exchange surface of the mechanical refrigeration circuit; and controllably flowing liquid nitrogen into the supplemental heat exchanger to further cool, by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, said atmosphere that has been cooled by heat exchange with said mechanical refrigeration circuit, wherein said liquid nitrogen is flowed into the supplemental heat exchanger only when the cooling of the atmosphere in the enclosure that is provided by said mechanical refrigeration circuit without said further cooling at the maximum that can be provided by said circuit or is within a preset percentage below said maximum, whereby said product is refrigerated by contact with said atmosphere.

22. A method according to claim 21 wherein all of the liquid nitrogen flowed into the supplemental heat exchanger vaporizes to gaseous nitrogen in the supplemental heat exchanger.

23. A method according to claim 21 wherein some but not all of the liquid nitrogen flowed into the supplemental heat exchanger vaporizes to gaseous nitrogen in the supplemental heat exchanger.

24. A method according to claim 21 further comprising alternatingly passing through said supplemental heat exchanger liquid nitrogen from said source thereof to further cool said atmosphere within said enclosure, and heated gaseous nitrogen from a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, wherein said heated nitrogen flows repeatedly through said recycle line and melts ice formed on a surface of said supplemental heat exchanger.

25. A method according to claim 21 wherein there is no heat exchange between nitrogen in the supplemental heat exchanger and the refrigerant of the mechanical refrigeration circuit.

26. A method according to claim 21 wherein the liquid nitrogen flows into the supplemental heat exchanger only when the cooling of the atmosphere in the enclosure that is provided by said mechanical refrigeration circuit without said further cooling of the atmosphere is at the maximum that can be provided by said circuit or is within a preset 90% of that maximum.

27. A method according to claim 26 wherein said control mechanism enables said flow when it senses that the temperature of the atmosphere in the enclosure is higher than a temperature that corresponds to the cooling provided by said circuit being within 90% of said maximum.

28. A method according to claim 26 wherein said control mechanism enables said flow when it senses that the temperature of the atmosphere in the enclosure is higher than a predetermined temperature set point.

29. A method according to claim 21 wherein the control mechanism that enables said liquid nitrogen to flow controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

30. A method according to claim 21 comprising controllably flowing said liquid nitrogen with (a) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source, or (b) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger, or (c) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

31. A method according to claim 21 wherein the supplemental heat exchanger is added inside the enclosure.

32. A method according to claim 21 wherein the supplemental heat exchanger is added outside the enclosure.

33. A method according to claim 21 wherein the supplemental heat exchanger physically shares a wall of the enclosure.

34. A method according to claim 21 wherein all circulation of the atmosphere within the enclosure is provided by said refrigeration impeller.

35. A method according to claim 21 further comprising adding a supplemental impeller in said enclosure and wherein circulation of said atmosphere within the enclosure is provided by said refrigeration impeller and by said supplemental impeller.

36. A method according to claim 21 wherein the supplemental heat exchanger cools atmosphere in a portion of the enclosure to a temperature that is lower than the temperature to which the mechanical refrigeration circuit cools the atmosphere.

37. A method according to claim 21 further comprising intermittently melting ice that has formed on a surface of the supplemental heat exchanger.

38. A method of refrigerating a product, comprising providing the product in an enclosure that contains an atmosphere, providing a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which can be coupled to a vent that is open to the atmosphere outside the enclosure, wherein the atmosphere in the enclosure can be cooled by heat exchange contact with the supplemental heat exchanger, and cooling atmosphere in the enclosure by heat exchange with the supplemental heat exchanger; controllably flowing liquid nitrogen into the supplemental heat exchanger to cool said atmosphere in the enclosure by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, and alternatingly passing through said supplemental heat exchanger liquid nitrogen from said source thereof to further cool said atmosphere within said enclosure, and heated gaseous nitrogen from a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, wherein said heated nitrogen flows repeatedly through said recycle line into said supplemental heat exchanger and melts ice formed on a surface of said supplemental heat exchanger, whereby said product is refrigerated by contact with said atmosphere in the enclosure.

39. A method according to claim 38 wherein control mechanism is provided that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

40. A method according to claim 38 comprising controllably flowing said liquid nitrogen with (a) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source, or (b) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger, or (c) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

41. A method according to claim 38 wherein the supplemental heat exchanger is inside the enclosure.

42. A method according to claim 38 wherein the supplemental heat exchanger is outside the enclosure.

43. A method according to claim 38 wherein the supplemental heat exchanger physically shares a wall of the enclosure.

44. Apparatus for refrigerating a product, comprising an enclosure that contains an atmosphere, refrigeration apparatus that comprises a mechanical refrigeration circuit comprising within the enclosure an evaporator having a heat exchange surface in contact within the enclosure with the atmosphere, the mechanical refrigeration circuit also comprising a compressor, a condenser, and an expansion device, which are connected together with the evaporator in fluid communication so that circulation of a refrigerant through the circuit cools the heat exchange surface and can thereby cool the atmosphere within said enclosure; the refrigeration apparatus also including a refrigeration impeller that circulates atmosphere into contact with said heat exchange surface; and a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which can be coupled to a vent that is open to the atmosphere outside the enclosure, wherein the supplemental heat exchanger is coupled to the refrigeration apparatus so that atmosphere that has been cooled by contact with the heat exchange surface of

40 the mechanical refrigeration circuit can be flowed in heat exchange contact with the supplemental heat exchanger, and control mechanism that enables liquid nitrogen to flow from said source into the supplemental heat exchanger to further cool, by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, said atmosphere that has been cooled by heat exchange with said mechanical refrigeration circuit, and that enables said flow of liquid nitrogen into the supplemental heat exchanger only when the cooling of the atmosphere in the enclosure that is provided by said mechanical refrigeration circuit without said further cooling is at the maximum that can be provided by said circuit or is within a preset percentage below said maximum.

45. Apparatus according to claim 44 further comprising a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, and control mechanism that enables the flow of nitrogen to be alternated between a first mode in which nitrogen can flow from the outlet of the supplemental heat exchanger into said recycle line and not to said vent, and nitrogen from said recycle line that has been heated by said heater and not liquid nitrogen from said source thereof can flow into the inlet of said supplemental heat exchanger and the nitrogen from said recycle line can be recycled into said supplemental heat exchanger repeatedly, and a second mode in which nitrogen can flow from the outlet of said supplemental heat exchanger to said vent and not into said recycle line, and liquid nitrogen from said source and not nitrogen from said recycle line can flow into the inlet of said supplemental heat exchanger.

46. Apparatus according to claim 44 wherein there is no heat exchange between nitrogen in the supplemental heat exchanger and the refrigerant of the mechanical refrigeration circuit.

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47. Apparatus according to claim 44 wherein the control mechanism enables liquid nitrogen to flow into the supplemental heat exchanger only when the cooling of the atmosphere in the enclosure that is provided by said mechanical refrigeration circuit without said further cooling of the atmosphere is at the maximum that can be provided by said circuit or is within a preset 90% of that maximum.

48. Apparatus according to claim 47 wherein said control mechanism enables said flow when it senses that the temperature of the atmosphere in the enclosure is higher than a temperature that corresponds to the cooling provided by said circuit being within 90% of said maximum.

49. Apparatus according to claim 44 wherein the control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

50. Apparatus according to claim 44 wherein said control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger controls said flow by (a) monitoring the pressure of liquid nitrogen flowing from said source, or (b) monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger, or (c) monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

51. Apparatus according to claim 44 wherein the supplemental heat exchanger is inside the enclosure.

52. Apparatus according to claim 44 wherein the supplemental heat exchanger is outside the enclosure.

53. Apparatus according to claim 44 wherein the supplemental heat exchanger physically shares a wall of the enclosure.

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54. Apparatus according to claim 44 wherein all circulation of the atmosphere within the enclosure is provided by said refrigeration impeller.

55. Apparatus according to claim 44 that further comprises a supplemental impeller in said enclosure and wherein circulation of said atmosphere within the enclosure is provided by said refrigeration impeller and by said supplemental impeller.

56. Apparatus according to claim 44 wherein the supplemental heat exchanger is capable of cooling atmosphere in a portion of the enclosure to a temperature that is lower than the temperature to which the mechanical refrigeration circuit cools the atmosphere.

57. Apparatus according to claim 44 further comprising controllable mechanism that can melt ice that has formed on a surface of the supplemental heat exchanger.

58. Apparatus for refrigerating a product, comprising an enclosure that contains an atmosphere, a supplemental heat exchanger which has an inlet that can be coupled to a source of liquid nitrogen and an outlet which can be coupled to a vent that is open to the atmosphere outside the enclosure, wherein atmosphere in the enclosure can be cooled by heat exchange contact with the supplemental heat exchanger, and control mechanism that controllably enables liquid nitrogen to flow from said source into the supplemental heat exchanger to further cool said atmosphere by heat exchange with said liquid nitrogen and with gaseous nitrogen formed by vaporization of said liquid nitrogen, and that controllably enables said flow of liquid nitrogen into the supplemental heat exchanger, and a recycle line in fluid communication with said supplemental heat exchanger and having an inlet between the outlet of the supplemental heat exchanger and said vent and having an outlet between the inlet of the supplemental heat exchanger and said source of liquid nitrogen, and wherein said recycle line comprises an impeller that can impel flow of nitrogen in said recycle line and a heater that can heat nitrogen that is in said recycle line, and control mechanism that enables the flow of nitrogen to be alternated between a first mode in which nitrogen can flow from the outlet of the supplemental heat exchanger into said recycle

43 line and not to said vent, and nitrogen from said recycle line that has been heated by said heater and not liquid nitrogen from said source thereof can flow into the inlet of said supplemental heat exchanger and the nitrogen from said recycle line can be recycled into said supplemental heat exchanger repeatedly, and a second mode in which nitrogen can flow from the outlet of said supplemental heat exchanger to said vent and not into said recycle line, and liquid nitrogen from said source and not nitrogen from said recycle line can flow into the inlet of said supplemental heat exchanger.

59. Apparatus according to claim 58 wherein the control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

60. Apparatus according to claim 58 comprising (a) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source, or (b) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger, or (c) control mechanism that enables said liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.

61. Apparatus according to claim 58 wherein the supplemental heat exchanger is inside the enclosure.

62. Apparatus according to claim 58 wherein the supplemental heat exchanger is outside the enclosure.

63. Apparatus according to claim 58 wherein the supplemental heat exchanger physically shares a wall of the enclosure.

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