Patent Application
20110072837
The present invention relates generally to refrigeration systems and, more particularly, to refrigeration systems for use with ultra-low temperature freezers.
Refrigeration systems are known for use with freezers of the type known as “ultra-low temperature freezers” (“ULT's”), which are used to cool their interior storage spaces to relative low temperatures such as about −80° C. or lower, for example.
Known refrigeration systems of this type include two stages circulating respective first and second refrigerants. The first stage transfers energy (i.e., heat) from the first refrigerant to the surrounding environment through a condenser, while the second refrigerant of the second stage receives energy from the cooled space (e.g., a cabinet interior) through an evaporator. Heat is transferred from the second refrigerant to the first refrigerant through a heat exchanger that is in fluid communication with the two stages of the refrigeration system.
In refrigeration systems of the type described above, the heat exchanger may be of a single-pass, coiled type. Heat exchangers of this type, however, typically occupy large spaces to permit the desired type of heat exchange between the refrigerants. The space and orientation requirements for these heat exchangers force designers to place them alongside or behind the walls of the inner freezer chamber, in front of the outer freezer cabinet, typically taking up valuable cooling/storage space thus not available for cooling.
In addition to the above, the attainable efficiency of these heat exchangers is limited by the relatively small amount of insulation that can be arranged around them between the walls of the inner freezer chamber and the outer freezer cabinet. Specifically, a relative small amount of insulation is placed around a heat exchanger of a refrigeration system of this type in order to minimize the amount of cooling/storage space that is lost to the heat exchanger.
There is a need, therefore, for refrigeration systems for use with ultra-low temperature freezers that can operate with a relative large efficiency, and which permit maximization of the cooling/storage space of the freezer.
In one embodiment, a refrigeration system is provided for use with an ultra-low temperature freezer having a deck and a refrigerated cabinet supported above the deck. The system has a first refrigeration stage and a second refrigeration stage. The first stage defines a first fluid circuit for circulating a first refrigerant. The first stage has a first compressor, a condenser, and a first expansion device that is in fluid communication with the first fluid circuit. The second stage defines a second fluid circuit for circulating a second refrigerant. The second stage has a second compressor, a second expansion device, and an evaporator that is in fluid communication with the second fluid circuit. The system includes an insulated enclosure supported within the deck and a split-flow heat exchanger that is in fluid communication with the first and second fluid circuits and which is located within the insulated enclosure.
In one embodiment, the heat exchanger may have a plurality of stacked plates that define flow paths for the first and second refrigerants through the heat exchanger. In a specific embodiment, the heat exchanger is in the form of a brazed plate heat exchanger. The heat exchanger may be oriented within the insulated enclosure such that a longitudinal dimension thereof is oriented generally vertical. The first refrigerant may enter the heat exchanger proximate a lower portion thereof and exit the heat exchanger proximate an upper portion thereof, such that the first refrigerant flows generally in an upward direction within the heat exchanger. Additionally, or alternatively, the second refrigerant may enter the heat exchanger proximate an upper portion thereof and exit the heat exchanger proximate a lower portion thereof, such that the second refrigerant flows generally in a downward direction within the heat exchanger.
In specific embodiments, the heat exchanger is of a counter-flow type. Also, the first expansion device may be located within the insulated enclosure. Additionally, the first expansion device may include at least one of a capillary tube or a valve. The first refrigeration stage may have a first accumulator that is in fluid communication with the first fluid circuit. The first accumulator may, for example, be located within the insulated enclosure. The first refrigeration stage may have a first filter/dryer that is in fluid communication with the first fluid circuit and which is supported within the deck. The first filter/dryer is, in some embodiments, located outside of the insulated enclosure. The second expansion device may be located outside the insulated enclosure. Additionally or alternatively, the second expansion device may include at least one of a capillary tube or a valve. The second refrigeration stage, in specific embodiments, has a second accumulator that is in fluid communication with a second fluid circuit. The second accumulator may, for example, be located outside of the insulated enclosure. Alternatively, the second accumulator may be located inside the insulated enclosure.
In another embodiment, a refrigeration system is provided for use with an ultra-low temperature freezer having a deck and a refrigerated cabinet supported above the deck. The system includes a first refrigeration stage and a second refrigeration stage. The first stage defines a first fluid circuit for circulating a first refrigerant, with the first stage having a first compressor, a condenser, a first filter/dryer, a first expansion device, and a first accumulator that is in fluid communication with the first fluid circuit. The second stage defines a second fluid circuit for circulating a second refrigerant, and has a second compressor, a second filter/dryer, a second expansion device, an evaporator, and a second accumulator that is in fluid communication with the second fluid circuit. The system also includes an insulated enclosure that is supported within the deck, and a split-flow heat exchanger that is in fluid communication with the first and second fluid circuits. The heat exchanger, the expansion device, the first accumulator, and the second filter/dryer are located within the insulated enclosure.
In yet another embodiment, an ultra-low temperature freezer is provided having a deck, a refrigerated cabinet supported above the deck, and a refrigeration system that is in thermal communication with the refrigerated cabinet. The refrigeration system includes a first refrigeration stage defining a first fluid circuit for circulating the first refrigerant, with the first stage having a first compressor, a condenser, and a first expansion device that is in fluid communication with the first fluid circuit. The system also includes a second refrigeration stage that defines a second fluid circuit for circulating a second refrigerant, with the second stage having a second compressor, a second expansion device, and an evaporator that is in fluid communication with the second fluid circuit. An insulated enclosure of the refrigeration system is supported within the deck, and a split-flow heat exchanger is in fluid communication with the first and second fluid circuits and is located within the insulated enclosure.
In another embodiment, a method is provided for operating an ultra-low temperature freezer. The method includes circulating a first refrigerant through a first compressor, a condenser, and a first expansion device of a first stage of a refrigeration system. A second refrigerant is circulated through a second compressor, a second expansion device, and an evaporator of a second stage of the system. A stream of at least one of the first or second refrigerants is split within a deck of the freezer into a plurality of streams that are arranged relative to one or more streams of the other of the first or second refrigerants so as to exchange heat between the first and second refrigerants. The method includes rejoining the plurality of streams and supporting a refrigerated cabinet of the freezer above the deck.
The method, in a specific embodiment, includes directing the stream of the at least one of the first or second refrigerants along a plurality of generally parallel streams. The method may be such that splitting the stream of the at least one of the first or second refrigerants includes turbulently flowing the at least one of the first or second refrigerants. Additionally or alternatively, splitting the stream of the at least one of the first or second refrigerants includes directing the at least one of the first or second refrigerants along a plurality of parallel plates that are spaced from one another. The method may include flowing the first refrigerant in a generally upward direction and/or flowing the second refrigerant in a generally downward direction.
In a specific embodiment, the method includes accumulating a liquid form of the first refrigerant or a liquid form of the second refrigerant within an insulated enclosure that concurrently insulates the plurality of streams. The method may include expanding the first refrigerant or the second refrigerant within one of a valve or a capillary tube.
Accordingly, the system and related methods described herein, by having a heat exchanger in the deck of the freezer, allows the positioning of adequate quantities of insulation around the heat exchanger, thereby attaining higher efficiencies than observed with conventional ultra-low temperature freezers. Moreover, having the heat exchanger in the deck of the freezer allows for maximization of the cabinet interior space above the deck.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention.
With reference to the figures, and more specifically to
With reference to
The first stage 24 includes, in sequence, a first compressor 50, a condenser 54, and a first expansion device 58. A fan 62 directs ambient air across the condenser 54 through a filter 54a and facilitates the transfer of heat from the first refrigerant 34 to the surrounding environment 40. The second stage 26 includes, also in sequence, a second compressor 70, a second expansion device 74, and an evaporator 78. The evaporator 78 is in thermal communication with the interior 16c of cabinet 16 (
In operation, the second refrigerant 36 receives heat from the interior 16c through the evaporator 78 and flows from the evaporator 78 to the second compressor 70 through a conduit 90. An accumulator device 92 is in fluid communication with conduit 90 to pass the second refrigerant 36 in gaseous form to the second compressor 70, while accumulating excessive amounts of the same in liquid form and feeding it to the second compressor 70 at a controlled rate. From the second compressor 70, the compressed second refrigerant 36 flows through a conduit 96 and into the heat exchanger 44 thermally communicating the first and second stages 24, 26 with one another. The second refrigerant 36 enters the heat exchanger 44 in gas form and transfers heat to the first refrigerant 34 while condensing into a liquid form. In this regard, the flow of the first refrigerant 34 may, for example, be counter-flow relative to the second refrigerant 36, so as to maximize the rate of heat transfer. In one specific, non-limiting example, the heat exchanger 44 is in the form of a split-flow brazed plate heat exchanger, vertically oriented within the deck 14 (
With continued reference to
As discussed above, the first refrigerant 34 flows through the first stage 24. Specifically, the first refrigerant 34 receives heat from the second refrigerant 36 flowing through the heat exchanger 44, leaves the heat exchanger 44 in gas form through an outlet 44b thereof and flows along a pair of conduits 114, 115 towards the first compressor 50. An accumulator device 116 is positioned between conduits 114 and 115 to pass the first refrigerant 34 in gaseous form to the first compressor 50, while accumulating excessive amounts of the same in liquid form and feeding it to the first compressor 50 at a controlled rate. From the first compressor 50, the compressed first refrigerant 34 flows through a conduit 118 and into the condenser 54. The first refrigerant 34 in condenser 54 transfers heat to the surrounding environment 40 as it condenses from gaseous to liquid form, before flowing along conduits 122, 123, through a filter/dryer unit 126, and into the first expansion device 58, where the first refrigerant 34 undergoes a pressure drop. From the first expansion device 58, the first refrigerant 34 flows though a conduit 127 back into the heat exchanger 44, entering the same in liquid form.
With continued reference to
In this exemplary embodiment, the heat exchanger 44 is oriented generally vertically and further such that the first refrigerant 34 flows in a generally upward direction while the second refrigerant 36 flows in a generally downward direction. More specifically, the first refrigerant 34 enters the heat exchanger 44 proximate a lower portion thereof and exits the same proximate an upper portion of the heat exchanger 44. Similarly, the second refrigerant 36 enters the heat exchanger 44 proximate an upper portion thereof and exits the same proximate a lower portion of the heat exchanger 44. As discussed above, the first refrigerant 34 evaporates from a liquid to a gaseous form in heat exchanger 44, while the second refrigerant 36 condenses from a gaseous to a liquid form in heat exchanger 44.
In the exemplary embodiment of
Among the factors which one skilled in the art can use in deciding which components to include within enclosure 150 are the expecting operating temperature of the specific component under steady state operating conditions, taking into account the boiling points and other characteristics of first refrigerant 34 and second refrigerant 36, the desired temperature at which cabinet interior 16c is to be maintained, various operating pressures and similar factors. For example, in ULT freezers with an expected cabinet temperature of about −86° C. and certain common refrigerants, the heat exchanger 44 is expected to operate under steady state conditions at about −40° C. Exemplary refrigerants suitable for the presently described embodiments include refrigerants commercially available under the respective designations R404A for the first refrigerant 34, and a mixture of R290 and R508B for the second refrigerant 36. Moreover, in specific embodiments, the first and second refrigerants may be combined with an oil to facilitate lubrication of the respective compressors 50, 70. For example, and without limitation, the first refrigerant 34 may be combined with Mobil EAL Artic 32 oil and the second refrigerant 36 may be combined with Zerol 150 Alkylbenzene oil. In another aspect of the present disclosure, the precise arrangement of the components illustrated in the figures is intended to be merely exemplary rather than limiting.
As discussed above, the heat exchanger 44 of the embodiment of
Each of the respective volumes between adjacent plates 160 defines a chamber 164, 166, within which one of the refrigerants 34, 36 flows. Further, the chambers 164, 166 are arranged in alternating fashion i.e., such that two adjacent chambers 164, 166 respectively receive the flow of two different refrigerants 34, 36. Under normal conditions, it is expected that each channel 164 will have liquid first refrigerant 34 adjacent to its base which evaporates as first refrigerant 34 moves upwardly. Under normal conditions, it is expect that each channel 166 will have gaseous second refrigerant 36 adjacent to is top which condenses as second refrigerant 36 moves downwardly. The levels for liquid refrigerant and for mixed liquid/gas refrigerant may vary between channels 164 and channels 166 and may also vary among parallel channels 164 and among parallel channels 166. Controls (not shown) can be used to minimize the situations in which particular channels 164 or 166 are occupied either entirely gaseous refrigerant or entirely by liquid refrigerant, except during start-up of the system.
In one aspect of the exemplary heat exchanger 44, the shapes of the channels 160a on the plates 160 are chosen so as to facilitate the generation of turbulent flow within the heat exchanger 44, which in turn maximizes the level of heat transfer between the refrigerants 34, 36. For example, and without limitation, the channels 160a may be chevron-shaped or be formed as pleats of corrugated plates. As used herein, the term “split-flow” heat exchanger refers to a heat exchanger that splits at least one of the first or second refrigerant streams from a single stream into a plurality of streams that are eventually rejoined into a single fluid stream.
While the exemplary heat exchanger 44 is arranged to receive there through respective pluralities of streams of the first and second refrigerants 34, 36, it is contemplated that, alternatively, a different type of split-flow heat exchanger 44 may be arranged such that only one of the refrigerants 34 or 36 may flow in multiple streams relative to the other refrigerant 36 or 34, respectively. For example and without limitation, alternative split-flow heat exchangers 44 may take the form of tube-and-shell heat exchangers, fin-plate heat exchangers, or other types of heat exchangers arranged to permit the flow of at least one of the refrigerants 34, 36 in a plurality of streams in a counter-flow, cross-flow, or parallel-flow arrangement. The use of any of these alternative types of heat exchangers 44 is deemed to fall within the scope of the present disclosure. Further, the exemplary heat exchanger 44 illustrated in
While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant's general inventive concept.
