WO2008112554A1

WO2008112554A1 - Refrigeration system

Info

Publication number: WO2008112554A1
Application number: PCT/US2008/056233
Authority: WO
Inventors: Alexander Cohr Pachai; John Ritmann
Original assignee: Johnson Controls Technology Company
Priority date: 2007-03-09
Filing date: 2008-03-07
Publication date: 2008-09-18
Also published as: WO2008112591A2; WO2008112549A2; WO2008112569A3; WO2008112569A2; WO2008112549A3; WO2008112594A2; WO2008112568A2; WO2008112572A1; WO2008112594A3; WO2008112566A2; WO2008112593A1; WO2008112568A3; WO2008112591A3; WO2008112566A3

Abstract

A multistage refrigeration system having a first system circulating a first fluid through a compressor, condenser, and an evaporator and a second system circulating a second fluid through a compressor, condenser, evaporator, receiver, and a pump, where the first fluid in the evaporator of the first system exchanges heat with the second fluid in the condenser of the second system. The multistage refrigeration system also has a defrost circuit to control the temperature of the refrigerant entering the defrost compressor and entering the evaporator. A controller operates the defrost circuit of the multistage refrigeration system.

Description

REFRIGERATION SYSTEM CROSS-REFERENCE TO RELATED PATENT 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 systems and methods of defrosting evaporators in refrigeration circuits of multistage refrigeration systems.

[0003] A multistage refrigeration system (also referred to as a cascade refrigeration system or a multi-pressure refrigeration system) 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 the refrigerated cases and the freezer cases in a supermarket. A multistage refrigeration system can also be used to provide an evaporator temperature that is 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, such 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 attained if only a single system is used with conventional heat-rejecting or heat-absorbing loads. [0005] Through routine operation of a multistage refrigeration system, frost may accumulate on the evaporators. The frost may need to be regularly removed to permit the system to operate as designed. In some freezing applications, a defrost cycle may be required to remove the frozen product from the freezing unit. The operation of a defrost cycle can frequently have a negative impact on the operation of the system.

SUMMARY

[0006] The present invention relates to a multistage refrigeration system having a first stage system circulating a first refrigerant through a first compressor, a first condenser, and a first evaporator and a second stage system circulating a second fluid through a second compressor, a second condenser, a second evaporator, and a receiver. The first refrigerant in the first evaporator exchanges heat with the second refrigerant in the second condenser. The second stage system has a first flow control device, a heat exchanger, and a second flow control device. The first flow control device is normally closed and controls the flow of the second refrigerant from the receiver. The second flow control device is normally open to provide a return flow of the second refrigerant to the receiver from the second evaporator. The second refrigerant from the receiver is circulated through the heat exchanger and supplied to the second evaporator when the first flow device is open and the second flow control device is closed.

[0007] The present invention also relates to a multistage refrigeration system having a first stage system circulating a first refrigerant through a first compressor, a first condenser, and a first evaporator and a second stage system circulating a second refrigerant through a second compressor, a second condenser, a second evaporator, and a receiver. The first refrigerant in the first evaporator exchanges heat with the second refrigerant in the second condenser. The second stage system has a heat exchanger and a flow control device receives fluid from the second compressor, A portion of the discharge gas from the second compressor is circulated through the heat exchanger and exchanges heat with the first refrigerant from the first evaporator. The discharge gas from the heat exchanger is supplied to the second evaporator through the flow control device for defrosting the second evaporator. [0008] The present invention further relates to a method for operating a multistage refrigeration system having the steps of drawing a second refrigerant from a receiver of a second stage system, directing the second refrigerant through a heat exchanger to exchange heat with a first refrigerant of a first stage system and directing the second refrigerant through an evaporator of the second stage system to defrost the evaporator.

BRIEF DESCRIPTION OF THE FIGURES

[0009] FIGS. 1 and 2 show exemplary embodiments of commercial and industrial applications incorporating a refrigeration system.

[0010] FIG. 3 shows a perspective view of an exemplary embodiment of a refrigeration system.

[0011] FIG. 4 shows a side elevational view of the refrigeration system shown in FIG. 3.

[0012] FIG. 5 schematically illustrates an exemplary embodiment of a multistage refrigeration system.

[0013] FIG. 6 schematically illustrates an exemplary embodiment of a multistage refrigeration system with a defrost circuit.

[0014] FIG. 7 schematically illustrates another exemplary embodiment of a multistage refrigeration system with a defrost circuit.

[0015] FIG. 8 schematically illustrates another exemplary embodiment of a multistage refrigeration system with a defrost circuit.

[0016] FIG. 9 schematically illustrates another exemplary embodiment of a multistage refrigeration system with a defrost circuit.

[0017] FIG. 10 schematically illustrates another exemplary embodiment of a multistage refrigeration system with a defrost circuit.

[0018] FIG. 11 schematically illustrates another exemplary embodiment of a multistage refrigeration system with a defrost circuit. DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0019] FIGS. 1 and 2 illustrate 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 system (also referred to as a high side system) and a second stage system (also referred to as a low side system) that are interconnected by a heat exchanger and 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.

[0020] FIG. 1 shows an application of an exemplary 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. According to an alternate exemplary embodiment, 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 yet another 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. According to another exemplary embodiment, 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.

[0021] 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 provide cooling temperatures of between about -20° C and about -50° C or colder and can be used when rapid freezing is desired 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 is used to assist in the removal of the product from between plates 30. FIGS. 1 and 2 illustrate exemplary applications only and multistage refrigeration systems are used in many other environments as well.

[0022] FIGS. 3 through 5 illustrate a multistage refrigeration system (shown schematically in FIG. 5). The multistage refrigeration system can include 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; for example, R-744A), ammonia (NH3; for example, R-717), hydrofluorocarbon (HFC) based refrigerants (for example, 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) chlorofluoro carbons (CFC) and hydrochlorofluorocarbons (HCFC), for example.

[0023] 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 or separator 52, a pump 54, and a first expansion device 56 and a first evaporator 58 that can be in parallel with a second device 60, such as a valve, and second evaporator 62. In another embodiment, second stage system can be operated with only first expansion device 56 and first evaporator 58. In still another embodiment, second stage system 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; for example, R-744), nitrous oxide (N2O; for example, R- 744A), blends of carbon dioxide and nitrous oxide, or hydrocarbon based refrigerants (for example, 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.

[0024] 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 10.

[0025] The condensed liquid refrigerant delivered to evaporator 46 in heat exchanger 36 enters into a heat exchange relationship with the fluid being circulated in condenser 50 in heat exchanger 36 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.

[0026] 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 critical pressure of the refrigerant, e.g., 73 bar at 31 deg C for carbon dioxide. Furthermore, during transcritical operation, the refrigerant is maintained as a single phase refrigerant (gas) in 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 transcritical 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-critical) that cools the refrigerant by heat exchange with another fluid. The cooling of the refrigerant gradually increases the density of the refrigerant. The refrigerant in the second stage can be the same or different than the refrigerant in the first stage. During transcritical 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.

[0027] 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 expansion device 56 and first evaporator 58 and to valve 60 and second evaporator 62 by pump 54.

[0028] 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 second expansion device 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. According to an exemplary embodiment, 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 fluid leaving second evaporator 62 may be a mixture of refrigerant vapor and refrigerant liquid. The refrigerant fluid exiting second evaporator 62, whether refrigerant vapor or a mixture of refrigerant vapor and refrigerant liquid, 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 condenser 50.

[0029] 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.

[0030] FIG. 6 illustrates an exemplary embodiment of a multistage system with a defrost circuit 64. The multistage system may include a first stage system 32 and a second stage 34 that are interconnected by heat exchanger 36. First stage system 32 can be a vapor compression system that circulates a refrigerant through a compressor 38, a condenser 40, a receiver 61, an expansion device 44, and an evaporator 46 incorporated in heat exchanger 36. Second stage system 34 can be a vapor compression system that circulates a refrigerant through a compressor 48, a condenser 50 incorporated in heat exchanger 36, a receiver 52, a pump 54, an expansion device 56 and a first evaporator 58 that may be supplied through second valve 60. [0031] Receiver 52 collects liquid refrigerant from condenser 50 through expansion valve 56 and the return line from evaporator 58 through valve 63. While FIG. 6 illustrates one evaporator 58, it is understood that more than one evaporator may be included in the second stage system. If more than one evaporator is disposed in the multistage refrigeration system, the return lines from the evaporators are connected into a single line that returns to receiver 52. Pump 54 supplies liquid refrigerant from receiver 52 to evaporator 58. Vapor refrigerant from receiver 52 can be circulated to compressor 48.

[0032] Defrost circuit 64, as shown in FIG. 6, is part of second stage system 34 and includes a heat exchanger 68 with a fan 65 to provide cooling to heat exchanger 68, a first valve 67, a second valve 69, valve 63 and valve 51. Any type of suitable flow control device may be used for valves 67, 69, 63 and 51. No pump is needed in defrost circuit 64, as self-circulation begins when evaporator 58 requires defrost. To initiate the defrost cycle for evaporator 58, valve 69 closes, valve 63 opens and valve 67 and valve 51 open simultaneously in defrost circuit 64 to provide the defrost circuit a flow path to circulate heated refrigerant to evaporator 58. Vapor refrigerant from defrost circuit 64 is heated in heat exchanger 68 and discharged into receiver 52, thereby heating the liquid refrigerant contained within receiver 52. Pump 54 circulates the heated refrigerant to the suction of evaporator 58 through valve 60. Refrigerant is discharged from evaporator 58 and circulated to evaporator 46. After evaporator 58 has been defrosted, second valve 69 closes, valve 63 opens and first valve 67 closes. Control of solenoid valves 67, 69 and 63 may be through a controller (not shown) or by manual operation.

[0033] FIG. 7 illustrates another exemplary embodiment of a multistage system with a defrost circuit 64. The multistage system may be similar to the embodiment described in FIG. 6 and can include first stage system 32 and second stage 34, which are interconnected by heat exchanger 36. Defrost circuit 64 receives discharged refrigerant from receiver 52 once the refrigerant has circulated through pump 54. By receiving discharged refrigerant after pump 54, the pressure in defrost circuit 64 is increased to provide higher temperature refrigerant to evaporator 58. First stage system 32 can be a vapor compression system that circulates a refrigerant through compressor 38, condenser 40, receiver 61, expansion device 44, and evaporator 46. Second stage system 34 can be vapor compression system that circulates a refrigerant through compressor 48, condenser 50 incorporated in heat exchanger 36, receiver 52, pump 54, expansion device 56 and first evaporator 58 that may be supplied by second valve 60.

[0034] Receiver 52 collects liquid refrigerant from condenser 50 incorporated in heat exchanger 36 and the return line from evaporator 58. While FIG. 7 illustrates one evaporator 58, it is understood that more than one evaporator may be included in second stage system 34. If more than one evaporator is disposed in the multistage refrigeration system, the return lines from the evaporators are connected into a single line that returns to receiver 52. Pump 54 supplies liquid refrigerant from receiver 52 to evaporator 58. Gas refrigerant from receiver 52 can be circulated to compressor 48.

[0035] Defrost circuit 64, as shown in FIG. 7, is part of second stage system 34 and includes heat exchanger 68, fan 65 to provide cooling to heat exchanger 68, valve 67, valve 69, valve 51 and valve 63. While solenoid valves 67, 69, 63 and 51 are described as being solenoid valves, any type of suitable flow control device may be used. No pump is needed in defrost circuit 64, as self-circulation begins when evaporator 58 requires defrost. To initiate the defrost cycle for evaporator 58, valve 69 closes, valve 63 opens and valve 67 and valve 51 open at approximately the same time in defrost circuit 64 to provide the defrost circuit a flow path to circulate heated refrigerant to evaporator 58. Pump 54 circulates refrigerant from the discharge of receiver 52 to heat exchanger 68, where the refrigerant is heated. By directing refrigerant from pump 54 to heat exchanger 68, the pressure of the refrigerant is higher, thereby increasing the refrigerant temperature for defrost circuit 64, Pump 54 also circulates the heated refrigerant to the suction of evaporator 58 through valve 60. Refrigerant is discharged from evaporator 58 and circulates to evaporator 46. When evaporator 58 has been defrosted, valve 69 closes, valve 63 opens and valve 67 closes. Control of valve 69, valve 63 and valve 67 in the defrost circuit may be through a controller (not shown) or manual operation.

[0036] FIG. 8 illustrates another exemplary embodiment of a multistage system with a defrost circuit 64. Defrost circuit 64 circulates vapor refrigerant from compressor 48 through heat exchanger 36, where heat is exchanged with vapor refrigerant from compressor 38. The heated vapor refrigerant circulates through evaporator 58 to defrost the evaporator. The multistage system may include a first stage system 32 and a second stage system 34 that are interconnected by a heat exchanger 36. First stage system 32 can be a vapor compression system that circulates a refrigerant through a compressor 38, a condenser 40, an expansion device 44, and an evaporator 46 incorporated within heat exchanger 36. Second stage system 34 can be a vapor compression system that circulates a refrigerant through a compressor 48, a first expansion device 49, a condenser 50 incorporated within heat exchanger 36, a receiver 52, a pump 54, an expansion device 56 and an evaporator 58 supplied by expansion device 57.

[0037] Defrost circuit 64 includes a heat exchanger 68. Heat exchanger 68 receives vapor refrigerant from compressor 48 and exchanges heat with the high temperature refrigerant discharged from compressor 38 from first stage system 32. Vapor refrigerant is discharged from heat exchanger 68 and supplied to evaporator 58 through valve 55 for defrost. Hot vapor phase defrost refrigerant is provided to evaporator 58 in a separate path and discharged to the suction of heat exchanger 36 and is discharged from evaporator 58 to heat exchanger 36 through valve 53. While FIG. 8 illustrates one evaporator, according to an exemplary embodiment, a plurality of evaporators may be included in the multistage refrigeration system and defrosted by defrost circuit 64.

[0038] FIG. 9 illustrates an exemplary embodiment of a multistage refrigeration system with a defrost circuit 64. Defrost circuit 64 includes defrost compressor 68 to increase the pressure and temperature of the discharge gas refrigerant from receiver 52 to defrost evaporator 58. The multistage refrigeration system can include a first stage system 32 and a second stage system 34 that are interconnected by heat exchanger 36. First stage system 32 can be vapor compression system that circulates a refrigerant through a compressor 38, a condenser 40, an expansion device 44, and an evaporator 46, which is incorporated within heat exchanger 36. Heat exchanger 36 receives refrigerant discharge from second stage system 34 from a compressor 48 and an evaporator 58. Vapor refrigerant discharge from evaporator 46 circulates to condenser 40, while liquid refrigerant discharge from condenser 50 flows to a receiver 52 located within second stage system 34. Second stage system 34 can be a vapor compression system that circulates a refrigerant through a compressor 48, a condenser 50, a receiver 52, a pump 54, an expansion device 56, and a first evaporator 58 supplied by an expansion device 71.

[0039] Defrost circuit 64 has a defrost compressor 68 and circulates discharge refrigerant from receiver 52 to provide heated refrigerant to evaporator 58 for defrost. Defrost compressor 68 receives hot vapor discharge from receiver 52, and compressor 68 increases the pressure of the gas refrigerant, thereby increasing the temperature of the refrigerant before entering evaporator 58. The refrigerant defrosts evaporator 58 in a separate defrost path, exchanging heat with discharge pumped from receiver 52 by pump 54. Discharge vapor from evaporator 58 enters the suction of receiver 52. Discharge vapor from receiver 52 enters compressor 48 before being discharged to heat exchanger 36 where the refrigerant exchanges heat with refrigerant from condenser 40 of first stage system 32. While one evaporator is shown in the second stage, according to an exemplary embodiment, a plurality of evaporators may be used in the multistage refrigeration system and defrosted by the defrost circuit.

[0040] FIG. 10 illustrates another embodiment of an exemplary defrost circuit 64 in a multistage refrigeration system, where a portion of the discharge from compressor 38 is diverted to the discharge of defrost compressor 68 before flowing to evaporator 58. According to an exemplary embodiment, a plurality of evaporators may be included that are defrosted by the defrost circuit. An alternate return line from evaporator 58 may provide discharge from evaporator 58 of second stage 34 to condenser 40 of first stage 32 through valve 51. The alternate return line and the direct discharge of defrost compressor 68 to evaporator 58 increases the pressure and temperature of the refrigerant. The increased pressure and temperature are intended to increase the efficiency of defrost circuit 64.

[0041] FIG. 11 illustrates another embodiment of an exemplary defrost circuit 64 in a multistage refrigeration system, where defrost compressor 68 provides heated refrigerant to multiple evaporators, or loads 80, 82, 84, 86 to defrost evaporators 80, 82, 84, 86. Compressor 68 compresses the heated refrigerant. A controller (not shown) determines which evaporator requires defrosting and opens a valve 90, 92, 94, 96 to defrost the particular evaporator. For example, if the controller (not shown) determines that evaporator 80 should be defrosted, valve 90 opens and heated refrigerant flows from defrost compressor 68 through valve 90 to evaporator 80. The controller (not shown) may use controls logic to determine the evaporator that requires defrosting, or a timer (not shown) may be used to sequentially and regularly defrost a particular evaporator. The controller (not shown) may also utilize a timer (not shown) and controls logic to determine the defrost requirement of evaporators 80, 82, 84, 86. The heated refrigerant exchanges heat in evaporators 80, 82, 84, 86 and cools to a lower temperature and partially condenses, producing a mixture of gas and liquid refrigerant leaving evaporators 80, 82, 84, 86. The mixture circulates to a condenser 98 where the mixture is condensed before circulating to an internal heat exchanger 100 through valve 102 and valve 108. The condensed refrigerant is evaporated in heat exchanger 100 and circulated to defrost compressor 68. A portion of the condensed refrigerant may circulate through valve 104 back to condenser 98, and a portion of the condensed refrigerant may circulate through valve 106 to heat exchanger 36. In heat exchanger 36, the compressed refrigerant exchanges heat with refrigerant from the refrigeration system (not shown) and evaporates before circulating to defrost compressor 68.

[0042] Defrost circuit 64 is shown as having four evaporators 80, 82, 84, 86 receiving defrost refrigerant from defrost compressor 68. Defrost compressor 68 may provide defrost refrigerant to more or less evaporators. Defrost circuit 64 may include more than one defrost compressor that may provide defrost refrigerant to additional evaporators. Defrost compressor may include a maximum capacity of about 52,0 bar for providing defrost refrigerant to evaporators 80, 82, 84 and 86. The refrigerant circulated in defrost circuit 64 in may be CO2 or any other suitable refrigerant.

[0043] Another embodiment of the multistage refrigeration system may include heat exchanger 36 being water cooled. The water may originate from any suitable source for example, wastewater. The multistage refrigeration system may include heat exchanger 36 being air cooled. The air for cooling heat exchanger 36 may originate from any suitable source, for example, a ventilation system.

[0044] While only certain features and embodiments of the invention have been illustrated and described, many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (e.g., temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, 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 description of the exemplary embodiments, all features of an actual implementation may not have been described (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 engineering 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, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.

Claims

CLAIMS:

1. A multistage refrigeration system comprising: a first stage system configured to circulate a first refrigerant through a first compressor, a first condenser, and a first evaporator; a second stage system configured to circulate a second fluid through a second compressor, a second condenser, a second evaporator, and a receiver; the first refrigerant in the first evaporator exchanging heat with the second refrigerant in the second condenser; and the second stage system comprising a first flow control device, a heat exchanger, and a second flow control device, the first flow control device being normally closed and configured to control flow of the second refrigerant from the receiver, the second flow control device being normally open to provide a return flow of the second refrigerant to the receiver from the second evaporator; wherein the second refrigerant from the receiver is circulated through the heat exchanger and supplied to the second evaporator when the first flow device is open and the second flow control device is closed.

2. The system of claim 1, wherein the heat exchanger exchanges environmental heat with the second refrigerant from the receiver to provide heated refrigerant to the second evaporator.

3. The system of claim 1, wherein the first flow control device and the second flow control device are controlled by a common controller.

4. The system of claim 1, wherein the heat exchanger is water-cooled.

5. The system of claim 1, comprising a plurality of valves being configured to open and close to control the circulation of defrost refrigerant to the second evaporator.

6. The system of claim 1, wherein the heat exchanger is air-cooled.

7. The system of claim 1, wherein the second refrigerant for the receiver is circulated through the heat exchanger and supplied to a plurality of evaporators.

8. The system of claim 1, wherein the second refrigerant in a gaseous phase is returned to the receiver after being discharged from the second evaporator.

9. The system of claim 1, wherein the first flow control device and the second flow control device are valves.

10. The system of claim 1, wherein a discharge line of the receiver is directed to a defrost compressor, thereby increasing pressure of the second refrigerant.

11. A multistage refrigeration system comprising: a first stage system circulating a first refrigerant through a first compressor, a first condenser, and a first evaporator; a second stage system circulating a second refrigerant through a second compressor, a second condenser, a second evaporator, and a receiver; the first refrigerant in the first evaporator exchanging heat with the second refrigerant in the second condenser; and the second stage system comprising a heat exchanger and a flow control device configured to receive fluid from the second compressor; wherein a portion of the discharge gas from the second compressor is circulated through the heat exchanger and exchanges heat with the first refrigerant from the first evaporator, and wherein the discharge gas from the heat exchanger is supplied to the second evaporator through the flow control device for defrosting the second evaporator.

12. The system of claim 11, wherein the flow control device is a valve.

13. The system of claim 12, wherein the valve is normally closed to prevent a flow of the discharge gas to the heat exchanger.

14. The system of claim 11, wherein the first condenser receives discharge from the second evaporator.

15. The system of claim 11, wherein the heat exchanger is water-cooled.

16. The system of claim 11, comprising a plurality of valves being configured to open and close to control the circulation of defrost refrigerant to the second evaporator.

17. The system of claim 11, wherein the heat exchanger is air-cooled.

18. The system of claim 11, wherein the second refrigerant for the receiver is circulated through the heat exchanger and supplied to a plurality of evaporators.

19. A method for operating a multistage refrigeration system, the method comprising the steps of: drawing a second refrigerant from a receiver of a second stage system; directing the second refrigerant through a heat exchanger to exchange heat with a first refrigerant of a first stage system; and directing the second refrigerant through an evaporator of the second stage system to defrost the evaporator.

20. The method of claim 19, comprising the step of directing the second refrigerant from the heat exchanger to a flow control device.