WO2008112572A1

WO2008112572A1 - Refrigeration system

Info

Publication number: WO2008112572A1
Application number: PCT/US2008/056287
Authority: WO
Inventors: Alexander Cohr Pachai; Saisunee Jommed; Jose Augusto Castro Chagas; Francisco Cazorla Marcos
Original assignee: Johnson Controls Technology Company
Priority date: 2007-03-09
Filing date: 2008-03-07
Publication date: 2008-09-18
Also published as: WO2008112549A3; WO2008112568A2; WO2008112549A2; WO2008112569A2; WO2008112554A1; WO2008112566A3; WO2008112566A2; WO2008112594A2; WO2008112594A3; WO2008112591A2; WO2008112568A3; WO2008112591A3; WO2008112569A3; WO2008112593A1

Abstract

A multistage refrigeration system having defrost circuit where a second refrigerant is drawn from an intermediate point of a condenser of a second stage system, directed through a heat exchanger to exchange heat with a first refrigerant of a first stage system and directed through an evaporator of the second stage system to defrost the evaporator. Also, the system has a defrost circuit where a second refrigerant is drawn from an intermediate point of a condenser of a second stage system, directed through a heat exchanger to exchange heat with a second refrigerant of the second stage system in a liquid state and directed through an evaporator of the second stage system to defrost the evaporator.

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] 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.

[0005] Through routine operation of a multistage refrigeration system, frost may build up on the evaporators that may need to be regularly removed to permit the system to operate as designed. In some freezing applications, defrosting 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 that circulates a first refrigerant through a first compressor, a first condenser, and a first evaporator and a second stage system that circulates a second refrigerant through a second compressor, a second condenser having an intermediate point, and a second evaporator. 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 defrost compressor, where the heat exchanger exchanges heat between the second refrigerant in a gaseous phase from the second compressor and the second refrigerant in a gaseous phase from an intermediate point of the second condenser. The defrost compressor supplies the second refrigerant to the second evaporator to defrost the second evaporator. The second refrigerant from the intermediate point of the second condenser is discharged from the heat exchanger to the defrost compressor.

[0007] The present invention also relates to a multistage refrigeration system having a first stage system that circulates a first refrigerant through a first compressor, a first condenser, and a first evaporator and a second stage system that circulates a second refrigerant through a second compressor, a second condenser having an intermediate point, and a second evaporator. 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 defrost compressor. The heat exchanger exchanges heat between the first refrigerant in a liquid phase from the first condenser and the second refrigerant in a gaseous phase from an intermediate point of the second condenser. The defrost compressor supplies the second refrigerant to the second evaporator to defrost the second evaporator. The second refrigerant from the intermediate point of the second condenser is discharged from the heat exchanger to the defrost compressor.

[0008] The present invention further relates to a method for operating a multistage refrigeration system having the steps of drawing a second refrigerant from an intermediate point of a condenser 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 a further 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 safety valve in 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 an embodiment of a liquid separator in the heat exchanger of the multistage refrigeration system of FIG. 8,

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 can be used as refrigerants in first stage system 32 are carbon dioxide (CO2; for example, 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.

[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 can be used as refrigerants in second stage system 34 are carbon dioxide (CO2; for example, R- 744), nitrous oxide (N2O; for example, R-744A), or mixtures of carbon dioxide and nitrous oxide, or hydrocarbon based refrigerants (for example, R- 170). 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 36.

[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 to first expansion device 56 and first evaporator 58 and then to second expansion device 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.

[0029J 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, In another embodiment, multiple stages of compression may be used, where the first stage system may include two separate stages and the second stage system may include two separate stages. Another embodiment includes condenser 50 being disposed outside of heat exchanger 36.

[0030] FIG. 6 illustrates an exemplary embodiment of a multistage system with a defrost circuit 64. The multistage system may be a similar embodiment described with respect to FIGS. 3 through 5, and can include first stage system 32 and second stage system 34 that are interconnected by heat exchanger 36. First stage system 32 can be a vapor compression system that circulates a refrigerant through compressor 38, condenser 40, expansion device 44, and evaporator 46 incorporated in heat exchanger 36. Second stage system 34 can be a vapor compression system that circulates a refrigerant through compressor 48, first heat exchanger 66 that is shared with defrost circuit 64, condenser 50 incorporated in heat exchanger 36, receiver 52, pump 54, first expansion device 56 and first evaporator 58 that can be supplied by second expansion device 60.

[0031] Receiver 52 collects liquid refrigerant from condenser 50 incorporated in heat exchanger 36 and the return line from evaporator 58. While FIG. 5 illustrates the use of 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 refrigerant to receiver 52. Pump 54 supplies liquid refrigerant from receiver 52 to evaporator 58, Gaseous phase 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 that includes defrost compressor 68 and circulates refrigerant from heat exchanger 66 to a parallel circuit in evaporator 58 to defrost evaporator 58. Heat exchanger 66 receives gaseous phase refrigerant, discharged from compressor 48 to exchange heat with refrigerant drawn from an intermediate point in condenser 50. The gaseous phase refrigerant from compressor 48 has a lower temperature after passing through heat exchanger 66. The refrigerant vapor from the intermediate point in condenser 50 is heated by the exchange of heat with the refrigerant vapor from compressor 48 and enters the suction of defrost compressor 68 and is provided to evaporator 58 for defrost. The refrigerant from defrost compressor 68 defrosts evaporator 58 using a separate path and returns to condenser 50.

[0033] FIG. 7 illustrates an exemplary embodiment of a multistage system with defrost circuit 64. The multistage refrigeration system can include first stage system 32 and second stage system 34 that are interconnected by heat exchanger 36. First stage system 32 can be a vapor compression system that circulates a refrigerant through compressor 38, condenser 40, heat exchanger 70, expansion device 44, and evaporator 46 incorporated in heat exchanger 36. Second stage system 34 can be a vapor compression system that circulates a refrigerant through compressor 48, condenser 50 incorporated in heat exchanger 36, first receiver 52, second receiver 72, pump 54, first expansion device 56 and evaporator 58 that can be supplied by a valve 60.

[0034] Second receiver 72 collects liquid refrigerant from receiver 52 and the return line from evaporator 58. While FIG. 7 illustrates the use of one evaporator 58, it is understood that more than one evaporator may be included in the system. If more than one evaporator is included in the multistage refrigeration system, the return lines from the evaporators can be collected into a single line that returns refrigerant to second receiver 72. Pump 54 supplies liquid refrigerant from second receiver 72 to evaporator 58. Gaseous phase refrigerant from second receiver 72 is provided to compressor 48 for circulation through second stage system 34.

[0035] Defrost circuit 64 includes defrost compressor 68 and circulates refrigerant from heat exchanger 70 to evaporator 58. Heat exchanger 70 receives liquid refrigerant discharged from condenser 40 and exchanges heat with gaseous phase refrigerant drawn from an intermediate point in condenser 50. The liquid refrigerant from condenser 40 exits heat exchanger 70 and flows to expansion device 44 and into evaporator 46 incorporated into heat exchanger 36. The refrigerant vapor from an intermediate point in condenser 50, after passing through heat exchanger 70, enters the suction of defrost compressor 68 and is provided to evaporator 58 for defrost. The refrigerant vapor from defrost compressor 68 returns to heat exchanger 36. [0036] FIG. 8 illustrates an exemplary embodiment of a multistage system with a defrost circuit 64. The multistage refrigeration system can include first stage system 32 and second stage system 34 that are interconnected by heat exchanger 36. First stage system 32 can be a vapor compression system that circulates a refrigerant through compressor 38, condenser 40, heat exchanger 70, expansion device 44, and evaporator 46 incorporated in heat exchanger 36. Second stage system 34 can be a vapor compression system that circulates a refrigerant through compressor 48, condenser 50 incorporated in heat exchanger 36, receiver 52, pump 54, first expansion device 56, first evaporator 58, second valve 60, second evaporator 62 and a third expansion device 76.

[0037] Defrost circuit 64 includes defrost compressor 68 and circulates refrigerant gas from receiver 52 to evaporators 58, 62. Discharge from the defrost circuits of evaporator 58 and evaporator 62 can return to condenser 48. Liquid refrigerant collected at the bottom of receiver 52 is supplied to evaporator 58 and evaporator 62 by pump 54 for cooling. Gas refrigerant can be discharged from receiver 52 to compressor 48. While FIG. 8 illustrates the use of evaporator 58 and evaporator 62, it is understood that more or less than two evaporators may be included in the system.

[0038] FIG. 9 illustrates an exemplary embodiment of a multistage system with a safety by-pass valve 80 and pressure sensor 82 to by-pass heated refrigerant in vapor from the discharge of compressor 48 through by-pass valve 80 and into the suction of compressor 48. High-pressure multistage systems operate near triple point, the pressure level that maintains refrigerant at a vapor, liquid, and solid state simultaneously. Such triple point systems are capable of forming dry ice within the piping and components when the pressure level drops below a predetermined level. The formation of dry ice within compressor 48 is harmful to compressor 48 and can cause damage to compressor 48.

[0039] The multistage refrigeration system shown in FIG. 9 can include first stage system 32 and second stage system 34 that are interconnected by heat exchanger 36. First stage system 32 can be a vapor compression system that circulates a refrigerant through compressor 38, condenser 40, heat exchanger 70, expansion device 44, and evaporator 46 incorporated in heat exchanger 36. Second stage system 34 can be a vapor compression system that circulates a refrigerant through compressor 48, condenser 50 incorporated in heat exchanger 36, receiver 52, pump 54, first expansion device 56, first evaporator 58, second valve 60, second evaporator 62 and a third expansion device 76.

[0040] Defrost circuit 64 includes defrost compressor 68 and circulates refrigerant gas from receiver 52 to evaporator 58. Discharge from the defrost circuits of evaporator 58 can return to compressor 48. Liquid refrigerant collected at the bottom of receiver 52 is supplied to evaporator 58 and evaporator 62 by pump 54 for cooling. Gas refrigerant can be discharged from receiver 52 to compressor 48. While FIG. 7 illustrates the use of evaporator 58 and evaporator 62, it is understood that more or less than two evaporators may be included in the system. Pressure sensor 82 monitors the pressure level of the refrigerant entering compressor 48. When the pressure level drops below a predetermined level, by-pass valve 80 opens to circulate hot refrigerant from the discharge of compressor 48 to the suction of compressor 48 to prevent the build up of dry ice in compressor 48.

[0041] FIG. 10 illustrates an exemplary embodiment of a multistage system with a defrost circuit 64. 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 a vapor compression system that circulates a refrigerant through a compressor 38, a condenser 40, an expansion device 44, and an evaporator 46 incorporated in heat exchanger 36. Second stage system 34 can be 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, an evaporator 58 and a second expansion device 60.

[0042] Defrost circuit 64 has a defrost compressor 68 and circulates refrigerant gas from heat exchanger 36 to evaporator 58. Discharge from the defrost circuits of evaporator 58 can return to compressor 48. Liquid refrigerant collected at the bottom of receiver 52 is supplied to evaporator 58 by pump 54 for cooling. Gas refrigerant can be discharged from receiver 52 to compressor 48. While FIG. 10 illustrates the use of evaporator 58, it is understood that more than one evaporator may be included in the system. [0043] In another embodiment, a liquid separator 74 may be disposed in defrost circuit 64 to substantially prevent excess liquid refrigerant vapors from entering the suction line of defrost compressor 34 as shown in FIG. 10. Liquid separator 74 may be a mesh to trap any excess liquid refrigerant vapors entrained in the gas refrigerant being drawn from heat exchanger 36 into the suction line of defrost compressor 34. A baffle or other similar shield 78 may be disposed in heat exchanger 36 to direct liquid refrigerant to the bottom of heat exchanger 36 to condenser 50. Gas refrigerant is drawn around the baffle 78 and supplied to defrost compressor 68. As shown in FIG. 1 1, liquid separator 74 may be disposed inside heat exchanger 36 to trap liquid refrigerant vapors from entering the suction of defrost compressor 34. Other exemplary embodiments may include liquid separator 74 being disposed outside of heat exchanger 36. Liquid separator 74 may be a mesh to trap any excess liquid refrigerant vapors entrained in the gas refrigerant being drawn from heat exchanger 36 into the suction line of defrost compressor 34.

[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 refrigerant through a second compressor, a second condenser having an intermediate point, and a second evaporator; the first refrigerant in the first evaporator exchanging heat with the second refrigerant in the second condenser; the second stage system comprising a heat exchanger and a defrost compressor, the heat exchanger being configured to exchange heat between the second refrigerant in a gaseous phase from the second compressor and the second refrigerant in a gaseous phase from an intermediate point of the second condenser, the defrost compressor being configured to supply the second refrigerant to the second evaporator to defrost the second evaporator, the second refrigerant from the intermediate point of the second condenser being discharged from the heat exchanger to the defrost compressor.

2. The system of claim 1, wherein the second condenser comprises a two-pass condenser.

3. The system of claim 1, comprising a liquid separator configured to remove liquid refrigerant from refrigerant from the intermediate point of the second condenser.

4. The system of claim 3, wherein the liquid separator comprises a mesh configured to entrain liquid refrigerant.

5. The system of claim 3, wherein the liquid separator is disposed inside the second condenser.

6. The system of claim 3, wherein the liquid separator is disposed outside of the second condenser.

7. The system of claim 1, wherein the heat exchanger is disposed inside the second condenser.

8. The system of claim 1, wherein the heat exchanger is disposed outside of the second condenser.

9. The system of claim 1 , wherein the second fluid comprises carbon dioxide.

10. The system of claim 1 , wherein the first fluid comprises ammonia.

1 1. The system of claim 1, wherein the second stage system comprises a by-pass valve and a pressure sensor, the by-pass valve being configured to open when the pressure sensor detects a pressure level of the second compressor dropping below a predetermined pressure level.

12. 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 refrigerant through a second compressor, a second condenser, and a second evaporator; the first refrigerant in the first evaporator exchanging heat with the second refrigerant in the second condenser; the second stage system comprising a heat exchanger and a defrost compressor, the heat exchanger being configured to exchange heat between the first refrigerant in a liquid phase from the first condenser and the second refrigerant in a gaseous phase from an intermediate point of the second condenser, the defrost compressor being configured to supply the second refrigerant to the second evaporator to defrost the second evaporator; the second refrigerant from the intermediate point of the second condenser being discharged from the heat exchanger to the defrost compressor.

13. The system of claim 12, wherein the second condenser is a two-pass condenser.

14. The system of claim 12, comprising a liquid separator configured to remove liquid refrigerant from refrigerant from the intermediate point of the second condenser.

15. The system of claim 14, wherein the liquid separator comprises a mesh configured to entrain liquid refrigerant.

16. The system of claim 14, wherein the liquid separator is disposed inside the second condenser.

17. The system of claim 14, wherein the liquid separator is disposed outside of the second condenser.

18. The system of claim 12, wherein the heat exchanger is disposed inside the second condenser,

19. The system of claim 12, wherein the heat exchanger is disposed outside of the second condenser.

20. The system of claim 12, wherein the second refrigerant comprises carbon dioxide.

21. The system of claim 12, wherein the first refrigerant comprises ammonia.

22. The system of claim 12, wherein the second stage system comprises a by-pass valve and a pressure sensor, the by-pass valve being configured to open when the pressure sensor detects a pressure level of the second compressor dropping below a predetermined pressure level.

23. A method for operating a multistage refrigeration system, the method comprising the steps of: drawing a second refrigerant from an intermediate point of a condenser 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.

24. The method of claim 23, comprising the step of directing the second refrigerant from the heat exchanger to a defrost compressor.

25. The method of claim 23, wherein the step of directing the second refrigerant through a heat exchanger further comprises directing the second refrigerant through a heat exchanger to exchange heat with a second refrigerant in a liquid stage from the second stage system.