Abstract:
A flash tank economizer includes a sensor for sensing a condition indicative of pressure in the flash tank, and when that pressure is found to equal or exceed the critical pressure of the particular refrigerant being used, a controller responsively closes a valve in the economizer vapor line to shut off the economizer. A sensor is also provided to sense the pressure at the compressor mid-stage, and if that pressure is found to exceed the pressure in the flash tank, the controller causes the flow control device to function so as to prevent the flow of refrigerant from the compressor mid-stage to the flash tank. Provision is also made for selectively draining refrigerant from the flash tank to reduce the pressure therein from a supercritical to a subcritical condition.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This PCT application claims priority to U.S. Provisional Patent Application No. 61/100,941, entitled “Flash Tank Economizer Cycle Control” filed Sep. 29, 2008 which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates generally to economized vapor compression systems and, more particularly, to a method and apparatus for controlling the flow within a flash tank economizer vapor line. 
       BACKGROUND OF THE INVENTION 
       [0003]    A vapor compression system consists of a compressor, a heat rejection heat exchanger or gas cooler, an expansion device, and an evaporator. Economizer cycles are sometimes employed to increase the efficiency and/or capacity of the system. Economizer cycles operate by expanding the refrigerant leaving the heat rejecting heat exchanger to an intermediate pressure and separating the refrigerant flow into two streams. One stream is sent to the heat absorbing heat exchanger, and the other is sent to cool the flow between two compression stages. In one form of an economizer cycle, a flash tank is used to perform the separation. In an economizer cycle with flash tank, a refrigerant discharged from the gas cooler passes through a first expansion device, and its pressure is reduced. Refrigerant collects in the flash tank as part liquid and part vapor. The vapor refrigerant is used to cool refrigerant exhaust as it exits a first compression device, and the liquid refrigerant is further expanded by a second expansion device before entering the evaporator. Such a flash tank economizer is particularly useful when operating in transcritical conditions, such as are required when carbon dioxide is used as the working fluid, and is described in U.S. Pat. No. 6,385,980, assigned to the assignee of the present invention. In the non-economized mode the vapor line connecting the flash tank with the compressor mid-stage is closed and the entire refrigerant mass flow rate entering the flash tank is directed to the second expansion stage. 
         [0004]    When the system operates in the economized mode, it is desirable to prevent the reversal of the flow direction in the economizer vapor line, e.g., from the compressor to the flash tank. That is, if the pressure in the compressor mid-stage is higher than in the flash tank, the flow direction in the economizer vapor line will be reversed, resulting in flow from the compressor through the economizer vapor line into the flash tank. Flow reversal in the economizer vapor line reduces the system cooling capacity and energy efficiency. Flow reversal will generally result when the compressor mid-stage pressure exceeds the pressure in the flash tank and can occur at certain operating conditions, dictated by the temperature at the heat sink and heat source and the specifics of the system design, such as heat exchanger size and compressor size. 
         [0005]    In U.S. Pat. No. 6,202,438, assigned to Scroll Technologies, a former subsidiary of the present assignee, there is disclosed an economized refrigeration circuit with a check valve disposed within the compressor to prevent the return flow of refrigerant from the compressor to the economizer. However, that check valve is employed only for that purpose, and a separate economizer valve is employed to turn the economizer on or off. Further, the economizer is not of the flash tank type, and the manner in which it operates is different from the flash tank economizer of the present invention. 
         [0006]    Due to the thermophysical properties of CO 2 , the refrigeration system can operate in both the subcritical and transcritical modes. The subcritical mode is similar to the operation of systems with conventional refrigerants. In the transcritical mode the refrigerant pressure in the heat rejection heat exchanger, and possibly in the flash tank, is above the critical pressure, while the evaporator operates as in the subcritical mode. If the flash tank pressure is above the critical pressure, the separation of the refrigerant into liquid and vapor phases will not occur as desired since a supercritical fluid does not form a distinct liquid and vapor phase. 
       DISCLOSURE OF THE INVENTION 
       [0007]    In accordance with one aspect of the invention, a flash tank economizer includes a control for preventing the operation of the economizer during periods in which the pressure in the flash tank is above the critical pressure of the refrigerant. 
         [0008]    In accordance with another aspect of the invention, the control is also responsive to the pressure difference between the flash tank and a mid-stage of the compressor so as to prevent operation of the economizer during periods in which the pressure at the mid-stage is greater than the pressure in the flash tank. 
         [0009]    In accordance with yet another aspect of the invention, provision is made to actively reduce the pressure in the flash tank when it is in the supercritical condition. 
         [0010]    In accordance with yet another aspect of the invention, provision is made to directly or indirectly measure pressure at a mid-stage of a compressor or pressure at a flash tank. 
         [0011]    In accordance with yet another aspect of the invention there is provided a vapor compression system of the type having in serial refrigerant flow relationship a compressor, a heat rejection heat exchanger, an expansion device and an evaporator, including a flash tank economizer disposed in serial flow relationship between the heat rejection heat exchanger and the expansion device, the flash tank economizer including a flash tank, a first flow control device disposed between the heat rejection heat exchanger and the flash tank, an economizer vapor line to fluidly interconnect the flash tank to a mid-stage of the compressor, a second flow control device disposed in the economizer vapor line, and a controller to control the second flow control device to prevent flow in the economizer line when pressure in said flash tank equals or exceeds the critical pressure of the refrigerant. 
         [0012]    In accordance with yet another aspect of the invention, there is provided a method of controlling the flow of refrigerant in a vapor compression system of the type having in serial refrigerant flow relationship a compressor, a condenser heat rejection heat exchanger, a first expansion device, a flash tank, a flow control device, a second expansion device and an evaporator, including fluidly interconnecting the flash tank to a mid-stage of the compressor by way of an economizer vapor line, providing a flow control device in the economizer vapor line, determining pressure in the flash tank, and responsively turning off the second flow control device to prevent flow in the economizer line when the pressure in the flash thank equals or exceeds the critical pressure of the refrigerant or when a mid-stage pressure of the compressor is greater than the pressure in the flash tank. 
         [0013]    In accordance with yet another aspect of the invention, there is provided a method of controlling the flow of refrigerant in a vapor compression system of the type having in serial refrigerant flow relationship a compressor, a heat rejection heat exchanger, a first expansion device, a flash tank, a flow control device, a second expansion device and an evaporator, including fluidly interconnecting the flash tank to a mid-stage of the compressor by way of an economizer vapor line, providing a flow control device in the economizer vapor line, determining pressure in the flash tank, and responsively turning off the second flow control device in the economizer line when the pressure in the flash thank equals or exceeds the critical pressure of the refrigerant or when a mid-stage pressure of the compressor is greater than the pressure in the flash tank. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic illustration of a vapor compression system with the present invention incorporated therein. 
           [0015]      FIG. 2  is a flow diagram showing the operation of the present invention. 
           [0016]      FIG. 3  is a schematic illustration of an alternative embodiment of the invention. 
           [0017]      FIG. 4  is a diagram graphically showing exemplary compressor mid-stage pressure as a function of compressor discharge pressure for various compressor suction pressures. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0018]    Shown in  FIG. 1  is a vapor compression system that includes, in serial flow relationship, a compressor  12 , a refrigerant heat rejection heat exchanger  13 , an expansion device  14 , and a heat absorption heat exchanger  16 . 
         [0019]    The compressor  12 , which functions to compress and circulate refrigerant through the refrigeration circuit, may comprise a single, multi-stage compressor having a lower compression stage  17  and higher compression stage  18  as shown and may comprise a scroll compressor, a screw compressor having stage compression pockets, a reciprocating compressor having at least a first bank of cylinders and a second bank of cylinders, or a multi-stage compressor. Alternatively, the compressor  12  may comprise a pair of single stage compressors connected in series refrigerant flow relationship. In one embodiment, the compressor  12  can comprise a scroll compressor or a multi-speed compressor (e.g., two-speed compressor). 
         [0020]    When the vapor compression system  11  is operating in a transcritical cycle, such as when charged with carbon dioxide refrigerant and operating at compressor discharge pressures in excess of the critical pressure point of carbon dioxide, the refrigerant heat rejection heat exchanger  13  operates at supercritical pressures and functions as a refrigerant vapor cooler, thus only cooling the refrigerant vapor and not condensing it to a liquid. The heat process of condensation will be described hereinbelow. 
         [0021]    The expansion device  14  may comprise an electrical expansion valve, a thermostatic expansion valve or a fixed orifice device, such as a capillary tube, all of which operate to expand the liquid refrigerant flowing to the expansion device  14  to a mixture of liquid and vapor as it enters the heat absorption heat exchanger  16 . 
         [0022]    The heat absorption heat exchanger  16 , commonly referred to as an evaporator, operates at a subcritical pressures and functions to cool a gas or liquid passing over the heat exchanger as the refrigerant therein is heated and evaporated. The heated vapor then passes to the inlet of the compressor  12 . 
         [0023]    Disposed in serial flow relationship between the heat rejection heat exchanger  13  and the expansion device  14  is a flow control device  19  and a flash tank  21 . The flow control device  19  and the flash tank  21 , together with an economizer vapor line  22  fluidly interconnecting the flash tank  21  to a mid-stage of the compressor  12 , comprise a flash tank economizer  23 . 
         [0024]    In operation, the refrigerant exiting the heat rejection heat exchanger  13  passes through the flow control device  19  where it is expanded to thereby reduce its pressure. The resulting mixture of liquid and vapor then enters the flash tank  21 , with the liquid  24  settling to the bottom and the vapor  26  residing in the top portion of the flash tank  21 . The liquid refrigerant  24  passes to the expansion device  14  where it is expanded as described hereinabove. 
         [0025]    In a process known as economized operation, the vapor  26  passes along the economizer vapor line  22  to a mid-stage point  27  of the compressor  12  to cool the refrigerant that exits the low compression stage  17  to thereby increase the cooling capacity of the system. Operation of such a flash tank economizer is described in greater detail in U.S. Pat. No. 6,385,980, assigned to the assignee of the present invention and incorporated herein by reference. 
         [0026]    Various problems arise with respect to use of such a flash tank economizer. First, if the pressure at the compressor mid-stage point  27  is greater than the pressure in the flash tank  21 , refrigerant will tend to flow from the compressor  12  to the flash tank  21 , resulting in a substantial reduction of system efficiency. Secondly, if the pressure in the flash tank  21  exceeds the critical pressure of the refrigerant (e.g., 1070 psia or 7.38 MPa for carbon dioxide), then the separation of liquid and vapor in the flash tank  21  will not occur as desired and the economizer will not function properly. Both of these problems can be addressed by way of a flow control device  28  placed in the economizer line  22  as shown. 
         [0027]    The flow control device  28 , which in one form is an electronically controlled flow control device such as a solenoid valve, is controlled by a controller  29  in response to sensed conditions at the flash tank  21  and at the compressor  12 . For example, a sensor S 1  senses an operational condition at the flash tank  21 , and a sensor S 2  senses an operational condition at the mid-stage point  27  of the compressor  12 . The sensed conditions then cause the controller  29  to either open the flow control device  28  to permit economized operation or to close the flow control device  28  to thereby turn off the economizer. 
         [0028]    In one embodiment, the sensor S 1  senses the pressure in the flash tank  21  and sends a signal along line  31  to the control  29 . The controller  29  then compares that sensed pressure with the critical pressure for the refrigerant being used, and if the sensed pressure is greater than the critical pressure, then the control  29  acts to close the flow control device  28 . 
         [0029]    In another embodiment, the sensor S 1  senses the temperature of the refrigerant in the flash tank  21 , with the temperature signal then being sent along line  31  to the controller  29 . If the controller  29  determines that the refrigerant temperature is below the critical temperature of the particular refrigerant (e.g. 31.1° C. or 88° F. for carbon dioxide), the flash tank pressure can be estimated from the corresponding refrigerant vapor pressure (this assumes that the refrigerant in the flash tank is in a two-phase state, which is a reasonable assumption for practical purposes), and then the flow control  28  will be responsively either placed in the open or close position as described hereinabove. 
         [0030]    In another embodiment, the operational condition (e.g., pressure) in the flash tank  21  and/or the operational condition (e.g., pressure) at the mid-stage point  27  of the compressor  12  can be indirectly sensed or calculated from other vapor compression system operational conditions. Accordingly, the pressure in the flash tank  21  can be determined by direct measurement (e.g., sensed by a sensor) or by indirect measurement (e.g., calculated by related parameters such as component characteristics or sensor readings). 
         [0031]    Recognizing the second problem as discussed hereinabove, the controller is also used for preventing the reverse flow of the refrigerant in the economizer vapor line  22 . That is, the sensor S 2  senses the pressure at the compressor mid-stage  27  and sends a pressure signal along line  32  to the controller  29 . The controller  29  then compares the pressure in the flash tank  21  with that at the compressor mid-stage  27 . If it is determined that the pressure at the compressor mid-stage  27  is greater than that in the flash tank  21 , the flow control device  28  is operated or closed such that the reverse flow cannot occur or is sufficiently reduced. 
         [0032]    An exemplary indirect determination for the compressor mid-stage pressure will now be described.  FIG. 4  shows the compressor mid-stage pressure as a function of the compressor discharge pressure for various compressor suction pressures. As shown in  FIG. 4 , the compressor mid-stage pressure can be determined when the suction and discharge pressure of the compressor  12  are known. The same information can be written in the form of an exemplary two-dimensional lookup table below. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 P Suction 1 
                 P Suction 2 
                 P Suction 3 
                 P Suction 4 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 P Discharge 1 
                 P Mid-Stage 1, 1 
                 P Mid-Stage 1, 2 
                 P Mid-Stage 1, 3 
                 P Mid-Stage 1, 4 
               
               
                 P Discharge 2 
                 P Mid-Stage 2, 1 
                 P Mid-Stage 2, 2 
                 P Mid-Stage 2, 3 
                 P Mid-Stage 2, 4 
               
               
                 P Discharge 3 
                 P Mid-Stage 3, 1 
                 P Mid-Stage 3, 2 
                 P Mid-Stage 3, 3 
                 P Mid-Stage 3, 4 
               
               
                 P Discharge 4 
                 P Mid-Stage 4, 1 
                 P Mid-Stage 4, 2 
                 P Mid-Stage 4, 3 
                 P Mid-Stage 4, 4 
               
               
                   
               
             
          
         
       
     
         [0033]    It should be understood that the values of the suction, discharge, and mid-stage pressures are specific to the compressor design and operating conditions. If the operating conditions for a given machine change, for instance if the suction superheat changes, the values of the mid-stage pressure for a particular combination of suction and discharge pressure may change. This is even more pronounced if the compressor design allows to independently control the speed of the two compressor stages, for instance if the two stages are driven by different motors, for which the speed can be adjusted independently from each other. In this case, an additional dimension can be added to the graph or lookup table. For example, an additional dimension can be accomplished by providing additional graphs or tables, each for a constant value of the additional variable. 
         [0034]    Referring now to  FIG. 2 , the process as performed by the control  29  is shown in block diagram form. In block  33 , the pressure at the flash tank is determined (e.g., sensed or calculated), and in block  34  that pressure is compared with the critical pressure for the particular refrigerant involved. If the flash tank pressure is less than the critical pressure, then the controller  29  proceeds to block  36 , and if the flash tank pressure is equal to or greater than the critical pressure, it proceeds to block  37 . 
         [0035]    In block  36 , the flash tank pressure is compared with the compressor mid-stage pressure from block  35 , and if it is greater than the compressor mid-stage pressure, then the controller proceeds to block  38  where the economizer vapor line  22  is opened. Again, the compressor mid-stage pressure can be directly or indirectly determined (block  35 ). If the flash tank pressure is not greater than the compressor mid-stage pressure, then the controller  29  proceeds to block  37 . If, at block  37 , a “no” signal is received from either block  34  or  36 , the economizer vapor line  22  is closed at block  39 . 
         [0036]    It should be recognized that the flow control device  28  may be of various types. For example, it may be an electronically controlled flow control device that is controlled in response to both the absolute flash tank pressure and the pressure difference between the flash tank pressure and compressor mid-stage pressure in order to perform the exemplary functions as described hereinabove. Alternatively, it may be an electronically controlled flow control device that responds only to the absolute flash tank pressure, and a separate flow control device such as a check valve, which is responsive to the pressure difference between the flash tank pressure and compressor mid-stage pressure so as to control or prevent flow in the reverse direction. It may also be a combined electronically controlled and directional flow control device (i.e., a combined solenoid and check valve), controlled according to both the flash tank pressure and by the pressure difference between the flash tank pressure and compressor mid-stage pressure. 
         [0037]    Referring now to  FIG. 3 , an alternative embodiment of the invention is shown wherein the flash tank pressure is actively controlled. That is, during periods in which the pressure in the flash tank is supercritical as, for example, during startup of the system at high ambient temperatures, the flash tank pressure can be reduced to subcritical conditions by draining some of the refrigerant mass (which may be in a vapor and/or liquid form) from the flash tank. This is accomplished by selectively fluidly interconnecting the economizer vapor line  22  to an inlet  41  of the lower compression stage  17  by way of a line  42  and flow control device  43 . Thus, when it is desired to reduce the pressure in the flash tank  21  from a supercritical condition, the flow control device  28  and the flow control device  43  are opened so as to allow a portion of the refrigerant from the flash tank  21  to drain into the inlet  41 . During this draining mode, the flow control device  44  is closed to prevent supercritical refrigerant from entering the compressor mid-stage  27 . After the pressure in the flash tank  21  has been reduced to a subcritical condition, the flow control device  43  may be closed and the flow control device  44  opened in order to permit operation to proceed as described hereinabove. 
         [0038]    It should be recognized that such a draining procedure may result in some liquid refrigerant entering the compressor inlet. Although this is generally undesirable, it may occur for short periods of time without any significant damage to the compressor. 
         [0039]    While the present invention has been described with reference to a number of specific embodiments, it will be understood that the true spirit and scope of the invention should be determined only with respect to claims that can be supported by the present specification. Further, while in numerous cases herein wherein systems and apparatuses and methods are described as having a certain number of elements it will be understood that such systems, apparatuses and methods can be practiced with fewer than the mentioned certain number of elements. Also, while a number of particular embodiments have been described, it will be understood that features and aspects that have been described with reference to each particular embodiment can be used with each remaining particularly described embodiment. For example, features or aspects described using  FIG. 1  or  FIG. 2  can be applied to embodiments described using  FIG. 3 .