Abstract:
The present invention describes a method and apparatus for controlling the operating pressure of a Reverse Brayton type refrigeration device  70  for the purposes of regulating cooling capacity and coolant inventory control.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a refrigeration system. In particular, this invention is related to a method and apparatus for controlling the operating pressure of a Reverse Brayton type refrigeration device for the purposes of regulating cooling capacity and coolant inventory control. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the food freezing industry, high food quality with low dehydration losses is obtained using low temperature liquid nitrogen freezing systems which operate at about −320° F. Ammonia and freon vapor compression mechanical systems, which operate at relatively high temperatures, such as −40° F., are commonly used to freeze food in an economical manner, but with high freezing times and high dehydration losses. Recently, high performance vapor compression, mechanical systems have emerged which produce high quality frozen foods with low dehydration losses at relatively high temperatures of from −40° F. to −60° F. Because they operate at such relatively high temperatures, dehydration losses associated with high performance mechanical freezers leave room for improvement. Common mechanical systems typically also cannot operate at low temperatures due to limitations associated with common refrigerants. If a low temperature refrigerant system could be developed, dehydration losses can be appreciably reduced. 
         [0003]    Refrigeration capacity control for refrigeration systems are routinely used and well known. They include rotating equipment (e.g., compressor/expanders) speed control and coolant loop pressure control. The use of reverse Brayton refrigerators for applications such as food freezing, as in the following arts. 
         [0004]    U.S. Pat. No. 5,524,442 discloses a cooling system includes a unit for processing product to be cooled or frozen. A secondary refrigeration loop is connected to this unit and introduces a refrigerant at or near atmospheric pressure into the unit. The secondary refrigeration loop may be open or closed. The secondary loop includes a secondary heat exchanger for cooling the refrigerant. A primary, closed refrigeration loop, operating at a pressure of not less than 2 atmospheres, includes a forward flow path which comprises a primary refrigerant compressor for producing compressed primary refrigerant, a primary heat exchanger for receiving and cooling the compressed primary refrigerant and an expander for further cooling and transferring the compressed refrigerant to the secondary heat exchanger to enable cooling of the secondary refrigerant. The primary loop further includes a return flow path from the secondary heat exchanger to the primary refrigerant compressor and to the primary heat exchanger. The primary heat exchanger thereby provides heat exchange from the return flow path to the forward flow path to accomplish a cooling action. 
         [0005]    U.S. Pat. No. 3,868,827 discloses a refrigeration system for freezing of food products, utilizing air as the working fluid. The system includes a freezing chamber having entrance and exit ports for passing a food product therethrough. A refrigerant supply main provides low temperature air to the freezer chamber, which air exits from the chamber via a return main. The warmed air passes through first and second stage compressors and aftercoolers positioned in line beyond each said compression stage. The high pressure output from the second stage compressor and aftercooler is also passed in countercurrent relationship through a heat exchanger, the cooler side of which carries the low pressure return flow of air refrigerant on its way to the first stage compressor. The compressed and cooled air from the compressor stages passes through an expansion turbine where the work performed thereby cools the air to the desired low temperature, and the cold air is then lead into the supply main leading back to the freezer chamber. A make-up and drier system, including a pair of drying beds, is connected to provide dry make-up air to the refrigerant return main to replace air lost at the product entrance and exit ports of the freezer chamber. 
         [0006]    U.S. Pat. No. 5,267,449 discloses a method and system for cooling air to cryogenic temperatures [e.g., below −100° F. (−730° C.)] for use as a refrigerant medium for direct contact cooling of articles such as foodstuffs for fast freezing. 
         [0007]    U.S. Pat. No. 5,483,806 discloses a refrigeration system using air as the refrigerant comprises a compressor which compresses air to 84 bar g. The compressed air is cooled first by cooling water and then by returning air in a plate-fin heat exchanger before being expanded to 59 bar g in an expander. The expanded air at −61° C. is passed through indirect cooling coils in a cold store which it leaves at −45° C. This air is then passed through the plate-fin heat exchanger before being recycled to the compressor. The refrigeration delivered is about 1.05 kw refrigeration/kw power input. 
         [0008]    Prior art refrigeration capacity control methods that use pressure control can be wasteful of coolant gas, requiring a buffer vessel, and/or have limited turndown services. Thus, there remains a need in the art for improved simple and effective refrigeration capacity control methods for providing pressure control. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention describes a method and apparatus for controlling the operating pressure of a Reverse Brayton type refrigeration device for the purposes of regulating cooling capacity and coolant inventory control. 
         [0010]    An embodiment of this invention is directed to a method for controlling the operating pressure of a refrigeration system, the method comprising: a) passing a cryogen through a closed loop reverse Brayton refrigerator; b) passing the cryogen through a condensation vessel; and c) passing the cryogen through a refrigerated space. The closed loop reverse Brayton refrigerator comprises a compressor, a chiller, a heat exchanger, and a turbine. The condensation vessel comprises a condensation coil therein, and is capable of condensing a cryogen. The cryogen may be many of the cryogen known to the skilled artisan. In one embodiment, the cryogen is liquid nitrogen. The cryogen that passes through the condensation coil is passed to vent. 
         [0011]    In another embodiment, the present invention is directed to a method for controlling the operating pressure of a refrigeration system, the method comprising a) passing a cryogen through a condensation vessel to form a gaseous cryogen to vent and a condensate to pass to a refrigerated space; b) passing the condensate from step a) through a heating element in a refrigerated space, and then passing the resulting heated condensate to a heat exchanger in a reverse Brayton refrigerator; c) passing the heated condensate from step b) to a compressor in the reverse Brayton refrigerator; d) passing the heated condensate from step c) to a chiller to form a cooled condensate; e) passing the cooled condensate from step d) to the heat exchanger in opposite flow from the flow in step b); f) passing the cooled condensate from step e) through a turbine to form a used condensate; and g) passing the used condensate from step f), which splits into a condensate for passing through the condensation vessel, and a condensate for joining the condensate from step a) into the heating element in the refrigerated space. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0012]    The above and other feature of the present invention will become more apparent by describing in detail exemplary embodiment thereof with reference to the attached drawings in which  FIG. 1  is a schematic illustration of the apparatus and method according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    Reverse Brayton type refrigerators are well known systems that can provide refrigeration through a sequence of compression, heat removal, and reversible de-compression. The reversible de-compression is typically through a turbo-expander and the overall cycle is often termed a Turbo-Brayton cycle. There are many modifications possible for these type of systems, including multi-stage compression and/or expansion and heat exchange processes. At least an embodiment of the present invention is best described using a simple example that employs the least mechanical complexity. It will be evident to those skilled in the art that numerous modifications and additions are possible within the scope of the present invention. 
         [0014]      FIG. 1  is a schematic of a simple, closed loop, Reverse Brayton refrigerator that is directed toward a refrigeration application (here shown as a simple heat exchanger inside an enclosure to be refrigerated). While not limited to any particular operating temperature, the present invention will be described in the context of a low temperature refrigeration application in the temperature range of about 0° C. to about −150° C. The typical working fluids for this temperature range are air, nitrogen, oxygen-nitrogen mixtures, or combinations of these gases as well as other gases such as argon and helium. 
         [0015]    For the system shown in  FIG. 1 , the closed loop cycle operates at a maximum pressure downstream of the compressor (P 1 ) and a minimum pressure downstream of the turbine (P 2 ). The overall cooling capacity of the system can be controlled during operation by a variety of techniques, including speed and pressure control. The technique of at least one embodiment is through pressure control. The pressure ratio (P 1 /P 2 ) is typically a constant value for a particular set of equipment, equipment speed, and refrigeration temperature (T). The amount of cooling is substantially a linear function of the absolute pressure of either P 1  or P 2 . That is, if P 2  (and, hence, P 1 ) is reduced by a factor of two, then the overall cooling produced is also reduced by a factor of two. Typical operating pressures (P 2 ) are between atmospheric (about 1 bara) and several hundred bar. Similarly, the pressure ratio may be from about 1.5 to 5. The higher pressure ratios will typically require multiple stage compression and/or expansion. 
         [0016]    Pressure control, at constant operating temperatures, implies that coolant (gas) mass must be added or removed. Typical prior art techniques include venting gas to reduce pressure, and introducing compressed gas to increase pressure. The vented gas can be lost, or may possibly be stored in a separate buffer volume. The gas introduced to increase pressure may either be new gas or gas from a buffer volume that may be further compressed to raise its pressure. Certain arrangements are possible where the gas is removed from the system from the high pressure side (P 1 ) into an intermediate pressure buffer vessel, then reintroduced when the pressure is to be raised into the low pressure side (P 2 ) without the need for a compressor. The problems associated with these techniques are that they can be wasteful of coolant gas, require additional compression equipment, require a large buffer vessel, and/or have limited turndown capability. 
         [0017]    The present invention provides a simple and effective means for providing pressure control, without introducing the limitations associated with prior art techniques. In an embodiment, the method is illustrated with an example where the operating fluid is nitrogen  10 , the operating pressure (P 2 )  54  is between about 1 and 20 bara, and the refrigeration temperature is about −50° C. to −150° C. 
         [0018]    Pressure control is achieved through the use of an attached condensation vessel  20  as shown in  FIG. 1 . Generally, in one embodiment, liquid cryogen  10  passes through first valve (V 1 )  12  into condensing coil  14  inside of the condensation vessel  18 . The liquid cryogen  10  emerges from condensation vessel  18  as gaseous cryogen  16  and is passed to vent  11 . 
         [0019]    Condensed cryogen  20  passes from condensation vessel  18  through second valve (V 2 )  22  as cryogen  24  for passage into refrigerated space  26 . Cryogen  60  combines with cryogen  24  to pass into heat exchange coils  28  in refrigerated space  26 . Condensed cryogen  30  passes from refrigerated space  26  and passes onto heat exchanger coil  32  in central heat exchanger  46 , emerging as cryogen stream  34 . Cryogen stream  34  passes onto compressor  36  emerging as cryogen stream  38 , which in turn passes through chiller  40  emerging as cryogen stream  42 . Cryogen stream  42  returns to heat exchanger coil  44  in central heat exchanger  46 , emerging as cryogen stream  48 . Cryogen stream  48  passes through first operating pressure (P 1 )  51  prior to passing onto turbine  52  and then second operating pressure (P 2 )  54 , emerging as cryogen stream  50 . Cryogen stream  50  splits into cryogen streams  58 ,  60 . Cryogen stream  58  recycles back onto condensation vessel  18 , and cryogen stream  60  recycles back onto refrigerated space  26  by joining with cryogen stream  24  for passage into refrigerated space  26 . In an embodiment, the part of the system containing legends  32  to  56  may be described as a Reverse Brayton Refrigerator. 
         [0020]    In an embodiment, for pressure reduction, liquid nitrogen, at a pressure lower than second operating pressure (P 2 )  54 , is introduced through first valve  12  into the condensing coil  14  inside the condensation vessel  18 . The relatively low pressure of the liquid nitrogen  10  will create a low temperature surface inside the condensation vessel  18  that is sufficiently cold to cause the higher pressure nitrogen gas at a second pressure (P 2 )  54  to condense. The rate of condensation will generally be proportional to the flow rate of liquid nitrogen  10 . Condensation is possible as long as the coolant gas is at a second pressure (P 2 )  54  below the critical pressure associated with the gas, which for nitrogen is 34 bara. The nitrogen  16  that is vented from the condensing coil  14  will be a gas at a temperature of about −150° C. to −196° C. The refrigeration capacity, on average, of this cold gas will likely be small compared to the amount of refrigeration being supplied by the Reverse Brayton refrigerator  70 . However, its cooling capacity may be usefully captured by simply introducing it directly or indirectly via a heat exchanger into the refrigerated space. 
         [0021]    The condensation vessel  18  will be insulated in order to prevent the condensed coolant fluid from rapidly vaporizing and raising the system pressure. In an embodiment, the condensation vessel  18  will be insulated with vacuum jacketing. To raise the system&#39;s second pressure (P 2 )  64 , some or all of the condensed coolant fluid is vaporized. While the vaporization can be achieved through a variety of heating techniques, in at least one embodiment, it may be accomplished by vaporizing the condensed coolant fluid  24  in the refrigerated space  26 . As shown in  FIG. 1 , this may be accomplished by opening second valve (V 2 )  22  and allowing the condensed liquid  24  to drain or be drawn into the heat exchanger  28  inside the refrigerated space  26 . The vaporization of the condensed liquid  30  is then easily accomplished while performing useful refrigeration. 
         [0022]    At least one embodiment of the present invention is to adjust the operating pressure, either (P 1 )  51  or (P 2 )  54 , according to the cooling demand required. The opening and closing of first and second valves, (V 1 )  12  and (V 2 )  22 , either lower or raise the operating pressure accordingly. An associated benefit to the present arrangement is ensuring the system pressure does not rise to an unacceptable level during periods on non-usage when the overall system temperature warms and the coolant gas expands. The condensation vessel  18  and associated pressure control will continue to perform whether or not the Reverse Brayton refrigerator  70  is operating. In addition, the overall inventory of coolant gas can easily be adjusted during periods when the refrigeration system is not operating. A simple technique is to pre-calibrate the level of condensed liquid in the condensation vessel that is present when the refrigerator is not operating and the system (outside the condensation vessel) is entirely at ambient temperature. The system pressure (P 1 )  51  and/or (P 2 )  54  can also be reduced during this time to a pressure sufficiently low by use of first valve (V 1 )  12  to allow introduction of relatively low pressure bulk gas (e.g., liquid or gaseous nitrogen). The point where replenishing gas or liquid is introduced can be anywhere in the system, but if it is liquid it would be preferably introduced directly into the condensation vessel  18 . 
         [0023]    In at least one embodiment, a variety of refrigeration cycles can employ this method of pressure control. In addition to recuperative cycles such as the Reverse Brayton, regenerative type cycles such as Stirling and Pulse Tube cycles can utilize the method of the present invention. For the Turbo-Brayton cycle, any number of compression and/or expansion stages may be employed. In addition, these compression/expansion stages may be coupled together for cycle optimization. 
         [0024]    In another embodiment, a wide variety of working fluids may be used, in addition to the normal atmospheric gases (oxygen, nitrogen, argon, air) any number of gases are possible. In addition, these gases may be combined to form specific mixtures with specific properties. 
         [0025]    In another embodiment, the orientation and arrangement of the condensation vessel may be modified in a number of ways, and the particular design of the condensing coil may be accomplished through any number of heat exchanger designs. 
         [0026]    In another embodiment, the method for re-vaporization of the condensed liquid, whether economized thermally within the system or not, may be changed. Including, for example, the use of electric or ambient vaporizers. 
         [0027]    In another embodiment, the use of the overall refrigeration system is not limited to providing refrigeration to a refrigerated space (e.g., a food freezer), but may by any number of cooling applications. This may also employ heat exchange designs that do not include a forced air circulation, but may, for example, use another intermediate heat exchange fluid. 
         [0028]    In yet another embodiment, the overall process refrigeration may include more than one refrigeration system, which may employ multiple refrigeration zones and sources. The use of the refrigeration may also be provided to multiple destinations, such as multiple food freezers or even different processes. 
         [0029]    In another embodiment, a method is provided for controlling the operating pressure of a refrigeration system, the method comprising: 
         [0000]    a) passing a gaseous first cryogen through a condensation vessel; cooling the gaseous first cryogen to condense some of said first cryogen into a liquid first cryogen and to establish a first pressure within said condensation vessel;
       passing any remaining gaseous first cryogen to a vent; and   passing said liquid first cryogen to a refrigerated space;
 
b) passing the first cryogen from step a) through a heating element in the refrigerated space;
   heating the first cryogen; and   passing the first cryogen to a regenerative heat exchanger within a reverse Brayton refrigerator, wherein said regenerative heat exchanger uses heated cryogen and chilled cryogen from the same reverse Brayton refrigerator for fluids which exchange heat therein;
 
c) passing the first cryogen from step b) to a compressor within the reverse Brayton refrigerator;
 
passing the first cryogen from step c) to a chiller;
   chilling said first cryogen;
 
e) passing the first cryogen from step d) to the regenerative heat exchanger;
 
f) passing the first cryogen from step e) through a turbine; and
 
g) passing a first fraction of the first cryogen from step f) to the refrigerated space, and passing a second fraction of the first cryogen from step f) to the condensation vessel.
       
 
         [0035]    In this method, the vent may be an output to the external atmosphere. Alternatively, the vent may output to a buffer vessel (not shown) capable of returning vented material to the system. As discussed above, the cryogen may be a gas selected from the group consisting of air; nitrogen; mixtures of oxygen and nitrogen; argon, helium; and combinations thereof. In one embodiment, the cryogen may consist essentially of nitrogen. According to this method, said first pressure within said condensation vessel may be adjusted to provide the cooling demand required. 
         [0036]    It will be understood that the embodiment(s) described herein are merely exemplary, and that one skilled in the art may make variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as described hereinabove. Further, all embodiments disclosed are not necessarily in the alternative, as various embodiments of the invention may be combined to provide the desired result.