WO2008112566A2

WO2008112566A2 - Refrigeration system

Info

Publication number: WO2008112566A2
Application number: PCT/US2008/056270
Authority: WO
Inventors: Alexander Cohr Pachai; John Ritmann; Istvan Knoll; Thomas Severin Christensen
Original assignee: Johnson Controls Technology Company
Priority date: 2007-03-09
Filing date: 2008-03-07
Publication date: 2008-09-18
Also published as: WO2008112568A3; WO2008112594A3; WO2008112593A1; WO2008112568A2; WO2008112554A1; WO2008112549A2; WO2008112566A3; WO2008112594A2; WO2008112569A2; WO2008112591A2; WO2008112549A3; WO2008112572A1; WO2008112569A3; WO2008112591A3

Abstract

A multistage refrigeration system is provided. The multistage refrigeration system includes a first stage system and a second stage system connected by a multi stage heat exchanger. A heat exchanger connected to either one of the first or second stage system is disposed within a receiver of the second stage system. In an exemplary embodiment, the heat exchanger is a first stage evaporator, such that the receiver functions to transfer heat between the stages of the multistage system and to accumulate a reservoir of liquid refrigerant for distribution through the second stage system.

Description

REFRIGERATION SYSTEM CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from and the benefit of U.S. Provisional Application No. 60/894,052, entitled SYSTEMS AND METHODS OF USING CO2 IN REFRIGERATION AND AIR CONDITIONING APPLICATIONS, filed March 9, 2007 and U.S. Provisional Application No. 60/917,175, entitled SYSTEMS AND METHODS OF USING NATURAL REFRIGERANTS, filed May 10, 2007, which are hereby incorporated by reference.

BACKGROUND

[0002] The application generally relates to refrigeration systems. The application relates more specifically to configurations for multistage refrigeration systems employing a receiver.

[0003] Multistage refrigeration systems (also referred to as cascade refrigeration systems or multi-pressure refrigeration systems) can be used when several evaporators are needed to provide various temperatures for a single application. For example, a multistage refrigeration system can be used to provide the necessary cooling for both refrigerated cases and freezer cases in a supermarket. A multistage refrigeration system can also be used to provide an evaporator temperature lower than that attainable by a single-stage system, e.g., a vapor compression system. For example, a multistage refrigeration system can be used in an industrial process to provide temperatures of between -20 deg C and -50 deg C or colder, as may be required in a plate freezer application.

[0004] One type of multistage refrigeration system can involve the interconnection of two or more closed loop refrigeration systems in which the heat- absorbing stage, e.g., evaporator, of one system is in a heat exchange relationship with the heat-rejecting stage, e.g., condenser, of the other system. One of the purposes of a multistage refrigeration system having the heat-absorbing stage of one system in a heat exchange relationship with the heat-rejecting stage of the other system is to permit the attaining of temperatures in the heat-rejecting or heat-absorbing stage of one of the systems that exceeds that which can be attainable if only a single system is used with conventional heat-rejecting or heat-absorbing loads. SUMMARY

[0005] The present invention relates to a multistage refrigeration system having a first stage system and a second stage system connected in a heat exchanger relationship. The first stage system includes a compressor, a condenser, a first stage expansion device and a first stage evaporator that are fluidly connected in a circuit The second stage system includes a second stage valve, a second stage evaporator, a receiver and a pump that are fluidly connected in a circuit The first stage evaporator is disposed within the receiver A first stage refrigerant flows through the first stage evaporator in a heat exchange relationship with a second stage refrigerant in the receiver Second stage refrigerant enteπng the receiver in a vapor state is condensed, the condensed second stage refrigerant accumulates in the receiver to form a liquid reservoir

[0006] The present invention also relates to a receiver for use in a multistage refrigeration system having first and second stage refrigerant systems The receiver includes a vessel containing a second stage refrigerant in a vapor state and a reservoir of the second stage refrigerant in a liquid state The vessel also includes a heat exchanger disposed inside the vessel The heat exchanger is fluidly connected to one of the first stage or the second stage of the multistage refrigeration system.

[0007] The present invention further relates to a multistage refrigeration system that includes a first stage and a second stage The first stage has a first stage compressor, a first stage condenser, a first stage expansion device and a first stage evaporator that are fluidly connected in a closed loop, the second stage includes a second stage compressor, a second stage condenser, a second stage valve device, a second stage evaporator, a receiver, and a pump that are fluidly connected in a second closed loop. A heat exchanger that is fluidly connected to either the first stage or the second stage is disposed in the receiver The multistage refrigeration system also has a multistage heat exchanger that includes the first stage evaporator and the second stage condenser, in the multistage heat exchanger, a first stage refrigerant flows m a heat exchange relationship with a second stage refrigerant BRIEF DESCRIPTION OF THE FIGURES

[0008] FIGS 1 and 2 show exemplary embodiments of commercial and industπal applications incorporating a refrigeration system

[0009J FIG 3 shows a perspective view of an exemplary embodiment of a refrigeration system.

[0010] FIG 4 shows a side elevational view of the refrigeration system shown in FIG 3

[0011] FIG 5 schematically illustrates an exemplary embodiment of a multistage refrigeration system

[0012] FIG 6 schematically illustrates an exemplary embodiment multistage refrigeration system m which a first stage evaporator is disposed m a second stage receiver

[0013] FIG. 7 schematically illustrates another exemplary embodiment of a multistage refrigeration system in which a first stage evaporator is disposed in a second stage receiver

[0014] FIG 8 shows an exemplary configuration of the second stage receiver

[0015] FIG 9 shows another exemplary configuration of the second stage receiver

[0016] FIG 10 schematically illustrates an exemplary embodiment in which a heat exchanger is disposed in the second stage receiver.

[0017] FIG 11 schematically illustrates another exemplary embodiment in which a heat exchanger is disposed in the second stage receiver

[0018] FIG 12 schematically illustrates yet another exemplary embodiment in which a heat exchanger is disposed in the second stage receiver

[0019] FIG. 13 schematically illustrates an exemplary embodiment with a recovery vessel positioned intermediate the second stage receiver and the second stage compressor DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0020] FIGS. 1 and 2 show several exemplary applications for a multistage refrigeration system (also referred to as a cascade refrigeration system or a multi- pressure refrigeration system). Multistage refrigeration systems can include a first stage refrigerant system (also referred to as a high side system) and a second stage refrigerant system (also referred to as a low side system) that are interconnected by a heat exchanger. Multistage refrigeration systems can be used to provide different levels of cooling capacity and/or achieve low temperatures that are difficult to achieve with a single vapor compression cycle.

[002 IJ FIG. 1 shows a multistage refrigeration system 10 that can provide both refrigeration and freezing capacity for a supermarket 12 in a commercial setting. The second stage system of multistage refrigeration system 10 can have evaporators incorporated into refrigerated cases or displays 14 and freezer cases or displays 16 that are accessible by a person shopping in supermarket 12. According to an exemplary embodiment, refrigerated cases or displays 14 can be used to keep produce or dairy products at a preselected temperature and can be operated at a temperature between about 2 deg C and about 7 deg C, and freezer cases or displays 16 can be used to keep frozen items at a preselected temperature and can be operated at a temperature between about -20 deg C and about -30 deg C. The second stage system of multistage refrigeration system 10 can have an evaporator 18 in a freezer storage area 20 of supermarket 12 and can have an evaporator 22 in a refrigerated storage area 24 of supermarket 12. According to an exemplary embodiment, freezer storage area 20 can be used to store items to be subsequently placed in freezer cases or displays 16 at a preselected temperature and can be operated at a temperature between about -20 deg C and about -30 deg C, and refrigerated storage area 24 can be used to store items to be subsequently placed in refrigerated cases or displays 14 at a preselected temperature and can be operated at a temperature between about 2 deg C and about 7 deg C.

[0022] FIG. 2 shows the use of a multistage refrigeration system 10 as a plate freezer 28 in a factory or industrial setting 26. Plate freezer 28 may have horizontal or vertical plates 30 to freeze flat products, such as pastries, fish fillets, and beef patties, as well as irregular-shaped vegetables that are packaged in brick-shaped containers, such as asparagus, cauliflower, spinach, and broccoli. The product may be firmly pressed between metal plates 30 that are cooled to subfreezing temperatures by internally circulating refrigerant from the second stage system through thin channels within plates 30 A high rate of heat transfer can be obtained between the product and plates 30. According to an exemplary embodiment, plate freezers 28 may provide cooling temperatures of between about -20 deg C and about -50 deg C or colder and can be used when rapid freezing is desired, for example, to retain product flavor and freshness. Once the product is frozen between plates 30, the product may be difficult to remove from plate freezer 28 because the product may be frozen to plates 30. A defrost system that warms plates 30 but does not thaw the product between plates 30 can be used to assist in the removal of the product from between plates 30. FIGS. 1 and 2 show exemplary applications only and multistage refrigeration systems are used in many other environments as well.

[0023] FIGS. 3 and 4 show a multistage refrigeration system 10 that is also shown schematically in FIG. 5, Multistage refrigeration system 10 includes a first stage system 32 and a second stage system 34 that are interconnected by a heat exchanger 36. Heat exchanger 36 can be a plate heat exchanger, a shell and tube heat exchanger, a plate and shell heat exchanger or any other suitable type of heat exchanger. First stage system 32 can be a vapor compression system that circulates a refrigerant through a compressor 38, a condenser 40, a receiver 42 (optional), an expansion device 44, and an evaporator 46 that is incorporated into heat exchanger 36. Some examples of fluids that may be used as refrigerants in first stage system 32 are carbon dioxide (CO2; e.g., R-744), nitrous oxide (N2O; e.g., R-744A), ammonia (NH3; e.g., R-717), hydrofluorocarbon (HFC) based refrigerants (e.g., R-410A, R-407C, R-404A, R- 134a), other low global warming potential (GWP) refrigerants, and any other suitable type of refrigerant, including hydrocarbons (HC) chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC), for example.

[0024] Second stage system 34 can be a vapor compression system that circulates a refrigerant through a compressor 48, a condenser 50 that is incorporated into heat exchanger 36, a receiver 52 (sometimes also referred to as a separator or pump separator when used in the low stage of a multistage refrigeration system), a pump 54, and a first expansion device 56 and a first evaporator 58 that can be in parallel with a valve 60 and second evaporator 62. According to another exemplary embodiment, second stage system portion 34 can be operated with only first expansion device 56 and first evaporator 58. According to still another exemplary embodiment, second stage system portion 34 can be operated as a volatile system by removing compressor 48, first expansion device 56 and first evaporator 58. Some examples of fluids that may be used as refrigerants in second stage system 34 are carbon dioxide (CO2; e.g., R-744), nitrous oxide (N2O; e.g., R-744A), blends of carbon dioxide and nitrous oxide, or hydrocarbon based refrigerants (e.g., R- 170). The refrigerant in the second stage can be the same or different than the refrigerant in the first stage. When second stage system 34 is operated as a volatile system, the refrigerant circulating through the system can be replaced with a glycol solution or a brine solution.

[0025] In first stage system 32, when operated sub-critically, i.e., below the critical pressure for the refrigerant being circulated in first stage system 32, compressor 38 compresses a refrigerant vapor and delivers the compressed vapor to condenser 40 through a discharge line. Compressor 38 can be a screw compressor, reciprocating compressor, centrifugal compressor, rotary compressor, swing link compressor, scroll compressor, turbine compressor, or any other suitable type of compressor. The refrigerant vapor delivered by compressor 38 to condenser 40 enters into a heat exchange relationship with a fluid, e.g., water from a cooling tower, and undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid. The condensed liquid refrigerant from condenser 40 can be stored in receiver 42 before flowing through expansion device 44 to evaporator 46 in heat exchanger 36.

[0026] The condensed liquid refrigerant delivered to evaporator 46 enters into a heat exchange relationship with the fluid being circulated in condenser 50 by second stage system 34, and undergoes a phase change to a refrigerant vapor as a result. The vapor refrigerant in evaporator 46 exits evaporator 46 and returns to compressor 38 by a suction line to complete the cycle.

[0027] First stage system 32 can be operated as a transcritical or supercritical system. During transcritical operation, first stage system 32 can be operated partly below (sub-critical) and partly above (supercritical) the critical pressure of the refrigerant circulated in first stage system 32. The discharge pressure of compressor 38 (or high side pressure) can be greater than the cπtical pressure of the refrigerant, e g , 73 bar at 31 deg C for carbon dioxide Furthermore, during transcπtical operation, the refrigerant is maintained as a single phase refrigerant (vapor phase) m the high pressure side of first stage system 32 and is first converted into the liquid phase when it is expanded in expansion device 44 When operated as a transcπtical system, the refrigerant from compressor 38 flows to a gas cooler (which can operate as a condenser in low ambient temperatures permitting the system to operate sub- cntical) that cools the refrigerant by heat exchange with another fluid The cooling of the refrigerant gradually increases the density of the refrigerant During transcπtical operation of first stage system 32, the high side pressure can be modulated to control capacity or to optimize the coefficient of performance by regulating the refrigerant charge and/or by regulating the total internal high side volume of refrigerant

[0028] In second stage system 34, compressor 48 compresses a refrigerant vapor and delivers the compressed vapor to condenser 50 through a discharge line Compressor 48 can be a screw compressor, reciprocating compressor, centrifugal compressor, rotary compressor, swing link compressor, scroll compressor, turbine compressor, or any other suitable type of compressor The refrigerant vapor delivered by compressor 48 to condenser 50 in heat exchanger 36 enters into a heat exchange relationship with the fluid being circulated in evaporator 46 by first stage system 32, and undergoes a phase change to a refrigerant liquid as a result. The condensed liquid refrigerant from condenser 50 is circulated to receiver 52 The liquid refrigerant in receiver 52 is circulated in parallel to first expansion device 56 and first evaporator 58 and to valve 60 and second evaporator 62 by pump 54

[0029] In first evaporator 58, the liquid refrigerant from first expansion device 56 enters into a heat exchange relationship with a cooling load, e g , a fluid, and undergoes a phase change to a refrigerant vapor as a result. The refrigerant vapor in first evaporator 58 exits first evaporator 58 and returns to compressor 48 to complete the cycle In second evaporator 62, the liquid refrigerant from valve 60 enters into a heat exchange relationship with a cooling load, e g , a fluid, and may undergo a phase change to a refrigerant vapor as a result However, some embodiments, the amount of refrigerant liquid provided to second evaporator 62 may exceed the heat exchange capabilities of the cooling load causing less than all of the liquid refrigerant to undergo a phase change. Thus, the refrigerant leaving second evaporator 62 may be a mixture of vapor phase and liquid phase refrigerant. The refrigerant exiting second evaporator 62, regardless of phase, returns to receiver 52. Receiver 52 can also have a connection to the discharge line from compressor 48 to provide refrigerant vapor from receiver 52 to the discharge line and subsequently to the condenser 50 in heat exchanger 36.

[0030] Compressor 38 of first stage system 32 and compressor 48 of second stage system 34 can each be driven by a motor or drive mechanism. The motor used with compressor 38 or compressor 48 can be powered by a variable speed drive (VSD) or can be powered directly from an alternating current (AC) or direct current (DC) power source. The VSD, if used, receives AC power having a particular fixed line voltage and fixed line frequency from the AC power source and provides power having a variable voltage and frequency to the motor. The motor used with compressor 38 or compressor 48 can be any type of electric motor that can be powered by a VSD or directly from an AC or DC power source. For example, the motor used with compressor 38 or compressor 48 can be a switched reluctance motor, an induction motor, an electronically commutated permanent magnet motor, or any other suitable motor type. In an alternate embodiment, other drive mechanisms such as steam or gas turbines or engines and associated components can be used to drive the motor used with compressor 38 or compressor 48.

[0031] Turning to FIG. 6, an embodiment is shown in which the multistage heat exchanger and the second stage receiver are combined into a single unit. First stage evaporator 46 is disposed in second stage receiver 52, such that the receiver functions as both the condenser and the receiver for the second stage system 200. In an exemplary embodiment, receiver 52 is a shell and tube heat exchanger.

[0032] FIG. 6 shows a single second stage evaporator 62. Vapor returning to receiver 52 from second stage evaporator 62 releases heat to first stage refrigerant flowing through first stage evaporator 46. The second stage vapor condenses and falls to the bottom of receiver 52 to sustain a liquid reservoir contained therein. Incorporating first stage evaporator 46 into second stage receiver 52 can eliminate the need for the multistage heat exchanger 36 as a separate unit, which can in turn reduce the overall size of the multistage refπgeration system 10, as well as decrease vibration within the system.

[0033] Receiver 52 is a vessel that contains a reservoir of liquid that feeds pump 54, which reservoir is sustained by the condensed vapor as well as any unvaporized liquid returning from the second stage evaporator(s) 62. Pump 54 supplies liquid refrigerant back to second stage evaporator 62 via valve 60. In one exemplary embodiment, valve 60 can be a regulation valve or a solenoid valve, in another exemplary embodiment, it can be an expansion valve; other valve types may also be employed. While any type of pump can be used, the pump is generally selected to have very low or no net positive suction head (NPSH). The use of a pump with very low or no NPSH can eliminate the need for a drop leg, which is a barrel-like tank at the low-pressure side of the pump sometimes used to ensure proper inflow conditions, but which can add additional cost and complexity.

[0034] In an exemplary embodiment, a portion of the liquid refrigerant leaving pump 54 is diverted back to receiver 52 through a minimum flow line in which a filter 64 is positioned. Filter 64 can serve the dual purpose of filtering sediment or other small particles from the refrigerant, as well as removing any water that may have been introduced or otherwise become entrained in the second stage. By placing the filter in the minimum-flow line, a smaller filter can be used and it can be changed without substantially impacting the overall operation of the multistage refπgeration system.

[0035] FIG 6 also shows the presence of an optional internal heat exchanger 66 in the first stage system 32 for superheating first circuit vapor flowing to compressor 38 and sub-cooling first circuit liquid flowing to evaporator 46. Superheating the first circuit vapor by about 10 deg C or more pπor to returning it to compressor 38 can eliminate the presence of liquid drops that may begin to form and which could damage compressor 38, In a similar manner, sub-cooling can prevent pre-mature vapoπzation of liquid refrigerant pπor to evaporator 46. The inclusion of the optional internal heat exchanger 66 in first stage system 32 may depend on the operating conditions of and/or refrigerant types used therein. For example, in an embodiment in which the first stage refrigerant is ammonia, superheating of the first stage refrigerant is generally not employed. [0036] In embodiments in which superheating of vapor in the first stage system is employed, an optional de-superheating circuit 68 may be employed. As shown in FIG. 6, the de-superheating circuit includes a de-superheater 70, which is a heat exchanger connected between compressor 38 and condenser 40. A fluid, such as ammonia, for example, flows through the de-superheater 70 in a heat exchange relationship with the superheated vapor leaving compressor 38. The warmed fluid in de- superheating circuit 6B is then cycled to a subsequent device 74 in need of heat energy, such as a hot water heater or defrost system, to release the heat absorbed in de-superheater 70. As a result, an environmental advantage can be achieved by recovering for use heat that would otherwise be wasted thereby offsetting energy demands of other devices.

[0037J FIG. 7 schematically shows another exemplary embodiment employing two second stage evaporators 58 and 62 connected in parallel, in which one of the evaporators 58 is part of a vapor compression loop in second stage system 34. In embodiments that employ a vapor compression loop in second stage system, an internal heat exchanger 76 may be included to superheat refrigerant vapor prior to second stage compressor 48 and to also subcool the liquid refrigerant leaving receiver 52. An optional de-super heater 78 may also be employed on the discharge side of second-stage compressor 48 in a second de -superheater circuit (not shown) to recover waste heat as described with respect to FIG. 6 regarding the first stage system 32. FIG. 7 shows an optional oil separator 80 incorporated in the second stage system 34 that can be employed to remove oil that is used to lubricate compressor 48 and that may have become entrained in the refrigerant. Receiver 52 may operate at very high pressures, such as 45 to 50 bar or higher. Thus, it is understood that although compressor 48 is shown schematically as a single compressor, one or more compressors in series can be used in the second stage system to reduce the load required of any single compressor unit. Similarly, one or more compressors in series can also be used in the first stage system.

[0038] FIG. 8 shows a schematic cross-sectional view of an exemplary configuration for receiver 52. Vapor and/or liquid refrigerant returning from second stage evaporators 58 and 62 is directed into receiver 52 by an inlet 82. Inlet 82 may be used for all returning vapor, or each second stage evaporator may enter the receiver via a separate inlet. The point of entry of refrigerant returning from second stage evaporators 58 and 62 is shown here as common inlet 82 that is positioned above the steady state level of liquid refrigerant forming the reservoir in receiver 52. In another embodiment, the inlet may be submerged in the reservoir, in that case, a vent (not shown) can be provided in the inlet to allow the refπgerant vapor to escape into the receiver pπor to submerging Vapor returning to receiver 52 from second stage evaporators 58 and 62 condenses and falls from the outer surface of first stage evaporator 46 as heat from the vapor is released to evaporate refπgerant flowing through first stage evaporator 46

[0039] FIG 8 shows first stage evaporator 46 as a flanged coil 84. The use of a flanged coil can allow better interchangeability and easier maintenance of evaporator 46 Flanged coil 84 can be for direct expansion or for dry expansion of any refrigerant of the first stage system and allows the amount of refπgerant in the first stage system to be reduced, which may result in both economical and environmental benefits.

[0040] FIG. 8 further shows a level sensor 86 positioned in receiver 52 Level sensor 86 is configured to detect if the reservoir of liquid refπgerant in receiver 52 decreases to a height that is below a predetermined level. Level sensor 86 can be integrated with a control network of the multistage refrigeration system and can provide an alarm and/or automatically stop pump 54 to avoid the pump running dry if level sensor 86 determines the reservoir has fallen below the predetermined level In some embodiments, pump 86 may be equipped with ceramic or synthetic beanngs to permit the pump to run dry for a limited period of time, adding a redundant level of safety

[0041| FIG. 9 shows an alternative embodiment in which receiver 52 further includes a standstill coil 88 disposed in receiver 52. The standstill coil may be arranged to be partially submerged in the liquid refngerant reservoir of the receiver or may be positioned above the steady state reservoir level. Standstill coil 88 is part of a separate standstill refπgeration unit 90 that can maintain the temperature within receiver 52, and thus prevent pressure buildup of carbon dioxide or other second stage refπgerant therein, for example, in the event of a power failure, compressor breakdown or other event that could cause the temperature and pressure in receiver 52 to rise. That is, the standstill unit 90 is provided to cool receiver 52 to hold down the pressure when the multistage refrigeration system is not operating (i.e., when it is at "standstill").

[0042] The standstill unit may be air or liquid (e.g., water or refrigerant) cooled and may incorporate a compressor (not shown) such as one suitable for a transcritical carbon dioxide or hydrocarbon refrigerant cycle. In another embodiment, the standstill unit may be a pulsed system using bottled nitrogen and/or carbon dioxide that can be vented to the atmosphere after passing through the standstill coil. To permit operation during a power failure, the standstill unit may be battery powered or connected to a generator, for example.

[0043] Other heat exchangers of multistage refrigeration system 10 can also be disposed in receiver 52 to take advantage of space saving and reduced levels of vibration that can thereby be accomplished. FIGS. 10 through 12 show exemplary embodiments in which one or more heat exchangers other than first stage evaporator 46 are disposed in receiver 52.

[0044] In FIG. 10, an arrangement is shown in which second stage system 34 employs a heat exchanger 92 disposed in receiver 52 for superheating vapor passing from evaporator 58 to compressor 48, FIG, 1 1 shows an arrangement in which two heat exchangers are disposed in receiver 52. As with FIG. 11 , internal heat exchanger 92 is disposed in receiver 52 for superheating vapor passing from evaporator 58 to compressor 48. A first stage heat exchanger 94 is also disposed in receiver 52 in which the first stage refrigerant flowing from condenser 40 to evaporator 46 in multistage heat exchanger 36 (shown here as a separate unit from receiver 52) is sub- cooled. The heat exchangers 92 and 94 may be as simple as one or more pipes that pass through receiver 52 if sufficient surface area can be provide for the heat exchange relationship; fins or other types of heat exchanger arrangements may also be employed to provide sufficient surface area. In some embodiments, the heat exchangers 92 and 94 may be plate exchangers.

[0045] When carbon dioxide is used as a refrigerant in the second stage, superheat can disappear more quickly, causing the gas to collapse resulting in liquid slugging in the compressor. In embodiments that include compressor 48 in second stage system 34, vapor can be drawn from receiver 52 and superheated to provide additional heat to vapor entering compressor 48 from second stage evaporator 58.

[0046] FIG. 12 shows an exemplary embodiment to accomplish this superheating in which liquid refrigerant returning to receiver 52 from second stage condenser 50 in multistage heat exchanger 36 flows first through a heat exchanger 96 disposed in receiver 52 before returning to the reservoir of receiver 52. Heat exchanger 96 is shown positioned in a turret 98 of receiver 52 that thereby effectively reduces the volume of vapor in receiver 52 providing a more effective superheating. The vapor is preferably superheated by at least about 10 deg C. This superheating reduces the likelihood that the vapor will collapse causing liquid to form in compressor 48. This embodiment also has the effect of sub-cooling the liquid returning to receiver 52 via heat exchanger 96. The sub-cooling results in reduced vapor generation when the liquid is expanded back into receiver 52.

[0047] FIG. 13 shows an alternative embodiment for avoiding liquid formation in the second stage compressor return line through the use of a recovery vessel 100, sometimes referred to as a slug pot. If vapor refrigerant leaving receiver 52 as suction gas has not been superheated, the vapor is more likely to condense as it flows to recovery vessel 100. As a result, the line flowing to recovery vessel 100 is preferably angled to allow any liquid that forms to flow toward recovery vessel 100 instead of back into receiver 52. A heat exchanger 102 is disposed in recovery vessel 100, which houses a flow of liquid refrigerant returning to receiver 52 from condenser 50. The vapor entering recovery vessel 100 from receiver 52 is super heated by the exchange of heat with the liquid refrigerant flowing through heat exchanger 102; liquid refrigerant entering recovery vessel 100 from receiver 52 is boiled off.

[0048] A baffle 104 can be positioned in recovery vessel 100 to increase the residence time of the vapor in recovery vessel 100 and increase the level of superheating. Because the liquid refrigerant entering recovery vessel 100 is generally boiled off, most of the liquid at the bottom of recovery vessel 100 is oil. The superheated vapor is pulled to compressor 48 from recovery vessel 100 via a return vent 106 that is in communication with a return tube 108 that ensures a controlled amount of oil is also returned to compressor 48 with the refrigerant. [0049J While only certain features and embodiments of the invention have been shown and descπbed, many modifications and changes may occur to those skilled in the art (e g , vaπations in sizes, dimensions, structures, shapes and proportions of the vaπous elements, values of parameters (e g , temperatures, pressures, etc ), mounting arrangements, use of materials, colors, oπentations, etc ) without materially departing from the novel teachings and advantages of the subject matter recited in the claims The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention Furthermore, in an effort to provide a concise descπption of the exemplary embodiments, all features of an actual implementation may not have been descπbed (i e , those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention) It should be appreciated that in the development of any such actual implementation, as in any engineenng or design project, numerous implementation specific decisions may be made Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabπcation, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue expeπmentation

Claims

WHAT IS CLAIMED IS:

1. A multistage refrigeration system comprising: a compressor, a condenser, a first stage expansion device and a first stage evaporator fluidly connected in a circuit to form a first stage system; and a second stage valve, a second stage evaporator, a receiver and a pump fluidly connected in a circuit to form a second stage system, the first stage evaporator being disposed within the receiver, wherein a first stage refrigerant flows through the first stage evaporator in a heat exchange relationship with a second stage refrigerant in the receiver, wherein second stage refrigerant entering the receiver in a vapor state is condensed, the condensed second stage refrigerant accumulating in the receiver to form a liquid reservoir.

2. The multistage refrigeration system of claim 1, wherein the second stage refrigerant is selected from the group consisting of carbon dioxide, nitrous oxide, and blends thereof.

3. The system of claim 2, wherein the first stage refrigerant is selected from the group consisting of carbon dioxide, nitrous oxide, and blends thereof.

4. The system of claim 2, wherein the first stage refrigerant is selected from the group consisting of ammonia, hydrocarbons, hydrofluorocarbons, hydrochlorofluorocarbons, chlorofluorocarbons, and blends thereof.

5. The system of claim 1, wherein the second stage comprises two second stage evaporators connected in parallel.

6. The system of claim 1, wherein the second stage further comprises a second stage compressor to compress vapor leaving the second stage evaporator.

7. The system of claim 6, wherein second stage refrigerant in a vapor state flows through a heat exchanger in a heat exchange relationship with second stage refrigerant in a liquid state.

8. The system of claim 7, wherein the second stage system further comprises a heat exchanger to remove heat from vapor leaving the second stage compressor.

9. The system of claim 1, wherein first stage refrigerant in a vapor state flows through a heat exchanger in a heat exchange relationship with first stage refrigerant in a liquid state.

10. The system of claim 9, wherein the first stage system further comprises a heat exchanger to remove heat from vapor leaving the first stage compressor.

1 1. The system of claim 1, wherein the receiver and the first stage evaporator are a shell and tube heat exchanger.

12. The system of claim 1, wherein the first stage evaporator comprises a flanged coil.

13. The system of claim 1, wherein the first stage evaporator is a plate heat exchanger.

14. The system of claim 1, wherein the receiver includes a level sensor configured to determine a height of the liquid reservoir in the receiver and further configured to turn off the pump in response to a determination that the height is below a predetermined level.

15. The system of claim 1, further comprising a refrigeration unit configured to maintain a temperature of the receiver below a predetermined temperature, wherein the refrigeration unit comprises a coil disposed in the receiver fluidly connected to a cooling source.

16. A receiver for use in a multistage refrigeration system having a refrigerant flowing through a first stage system in a first closed loop and a refrigerant flowing through a second stage system in a second closed loop, the receiver comprising: a vessel containing a second stage refrigerant in a vapor state and a reservoir of the second stage refrigerant in a liquid state; and a heat exchanger disposed inside the vessel, the heat exchanger being fluidly connected to one of the first stage or the second stage of the multistage refrigeration system.

17. The receiver of claim 16, wherein the heat exchanger functions as an evaporator of the first stage system when fluidly connected to the first stage system, the first stage further comprising a compressor, a condenser, and an expansion device.

18. The receiver of claim 16, wherein the heat exchanger is a plate heat exchanger.

19. The receiver of claim 16, wherein the vessel and the heat exchanger form a shell and tube heat exchanger.

20. The receiver of claim 16, wherein the heat exchanger comprises a flanged coil.

21. The receiver of claim 16, wherein the heat exchanger is fluidly connected intermediate a first stage system condenser and a first stage evaporator to subcool first stage liquid refrigerant flowing therethrough.

22. The receiver of claim 16, wherein the heat exchanger is fluidly connected intermediate a second stage evaporator and a second stage compressor to superheat second stage vapor refrigerant flowing therethrough.

23. The receiver of claim 16, wherein the heat exchanger is fluidly connected intermediate a second stage condenser and the vessel.

24. The receiver of claim 16, further comprising a second heat exchanger disposed in the vessel, wherein the second heat exchanger is fluidly connected to a refrigeration unit configured to deliver a cooling fluid through the second heat exchanger to maintain a temperature of the vessel below a predetermined temperature.

25. The receiver of claim 16 further comprising a level sensor disposed in the vessel, the level sensor configured to determine a reservoir level in the vessel and further configured to provide a signal for taking corrective action in response to a determination that the reservoir level is below a predetermined level.

26. A multistage refrigeration system comprising: a first stage having a first stage compressor, a first stage condenser, a first stage expansion device and a first stage evaporator fluidly connected in a closed loop; and a second stage having a second stage compressor, a second stage condenser, a second stage valve, a second stage evaporator, a receiver, and a pump fluidly connected in a second closed loop; a multistage refrigeration heat exchanger comprising the first stage evaporator and the second stage condenser, wherein, in the multistage heat exchanger, a first stage refrigerant flows in a heat exchange relationship with a second stage refrigerant; wherein the receiver comprises a heat exchanger disposed therein that is fluidly connected to either the first stage or the second stage.

27. The system of claim 26, wherein the heat exchanger disposed in the receiver is fluidly connected to the first stage.

28. The system of claim 27, wherein first stage refrigerant flows as a liquid through the heat exchanger disposed in the receiver.

29. The system of claim 26, wherein the heat exchanger disposed in the receiver is fluidly connected to the second stage.

30. The system of claim 29, wherein second stage refrigerant flows as a vapor through the heat exchanger disposed in the receiver.

31. The system of claim 29, wherein second stage refrigerant flows as a liquid through the heat exchanger disposed in the receiver.

32. The system of claim 31, the receiver further comprising a suction gas outlet fluidly connected to the second stage compressor, wherein second stage refrigerant vapor at the suction gas outlet of the receiver is superheated by second stage refrigerant flowing through the heat exchanger disposed in the receiver.

33. The system of claim 26, wherein the receiver comprises a first heat exchanger disposed in the receiver that is fluidly connected to the first stage and a second heat exchanger disposed in the receiver that is fluidly connected to second stage.

34. The system of claim 26, further comprising a recovery vessel intermediate a suction gas outlet of the receiver and the second stage compressor.

35. The system of claim 26, wherein the second stage refrigerant is selected from the group consisting of carbon dioxide, nitrous oxide, and blends thereof.