Abstract:
A flash tank employing valves for use in transcritical cycles of a vapor compression system to increase the efficiency and/or capacity of the system. Carbon dioxide is preferably used as the refrigerant. The high pressure of the system (gas cooler pressure) is regulated by controlling the amount of charge in the flash tank by actuating valves positioned on the expansion devices located at the entry and exit of the flash tank. If the pressure in the gas cooler is too high or too low, the valves can be adjusted to either store charge in or release charge from the flash tank. By regulating the amount of charge in the flash tank, the high pressure of the system can be controlled to achieve optimal efficiency and/or capacity.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a means for regulating the high pressure component of a transcritical vapor compression system. 
     Chlorine containing refrigerants have been phased out in most of the world due to their ozone destroying potential. Hydrofluoro carbons (HFCs) have been used as replacement refrigerants, but these refrigerants still have high global warming potential. “Natural” refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Unfortunately, there are problems with the use of many of these fluids as well. Carbon dioxide has a low critical point, which causes most air conditioning systems utilizing carbon dioxide as a refrigerant to run transcritical under most conditions. 
     When a vapor compression system is run transcritical, it is advantageous to regulate the high pressure component of the system. By regulating the high pressure of the system, the capacity and/or efficiency of the system can be controlled and optimized. Increasing the high pressure of the system (gas cooler pressure) lowers the specific enthalpy entering the evaporator and increases capacity. However, more energy is expended because the compressor must work harder. It is advantageous to find the optimal high pressure of the system, which changes as operating conditions change. By regulating the high pressure component of the system, the optimal high pressure can be selected. Hence, there is a need in the art for a means for regulating the high pressure component of a transcritical vapor compression system. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a means for regulating the high pressure component of a transcritical vapor compression system. 
     A vapor compression system consists of a compressor, a 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. This invention regulates the high pressure component of the vapor compression system (pressure in the gas cooler) by controlling the amount of charge in the flash tank. In a preferred embodiment of the invention, carbon dioxide is used as the refrigerant. 
     In a flash tank, refrigerant discharged from the gas cooler passes through a first expansion device, and its pressure is reduced. The 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. 
     Expansion valves positioned on the path leading into and out of the flash tank are used to expand the refrigerant from high pressure to low pressure. This invention controls the actuation of the expansion valves to control the flow of charge into and out of the flash tank, regulating the amount of charge stored in the flash tank. By regulating the amount of charge stored in the flash tank, the amount of charge in the gas cooler and the high pressure of the system can be controlled. 
     An optimal pressure of the system can be selected by controlling the actuation of the valves. If the pressure in the gas cooler is too low, the expansion valves can be adjusted to release charge from the flash tank into the system to increase the gas cooler pressure, increasing the capacity of the system. If the pressure in the gas cooler is too high, the expansion valves can be adjusted to store charge in the flash tank to decrease the gas cooler pressure, reducing the energy expended by the compressor. 
     Accordingly, the present invention provides a method and system for regulating the high pressure component of a transcritical vapor compression system. 
     These and other features of the present invention will be best understood from the following specification and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
     FIG. 1 illustrates a schematic diagram of a prior art vapor compression system. 
     FIG. 2 illustrates a thermodynamic diagram of a transcritical vapor compression system. 
     FIG. 3 illustrates a schematic diagram of a prior art two stage vapor compression system utilizing a flash tank. FIG. 4 illustrates a thermodynamic diagram of a two stage economized cycle and a noneconomized cycle of a transcritical vapor compression cycle. 
     FIG. 5 illustrates a schematic diagram of a flash tank of a two stage vapor compression system utilizing expansion valves to control the high pressure of the system. 
     FIG. 6 illustrates a schematic diagram of a two stage flash tank of a vapor compression system utilizing additional valves to control the high pressure of the system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the invention may be susceptible to embodiments in different forms, there is shown in the drawings, and herein will be described in detail, specific embodiments with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that as illustrated and described herein. 
     FIG. 1 illustrates a prior art vapor compression system  10 . A basic vapor compression system  10  consists of a compressor  12 , a heat rejecting heat exchanger (a gas cooler in transcritical cycles)  14 , an expansion device  16 , and a heat accepting heat exchanger (an evaporator)  18 . 
     Refrigerant is circulated though the closed circuit cycle  10 . In preferred embodiments of the invention, carbon dioxide is used as the refrigerant. While carbon dioxide is illustrated, other refrigerants may be used. Because carbon dioxide has a low critical point, systems utilizing carbon dioxide as a refrigerant usually require the vapor compression system  10  to run transcritical. 
     When the system  10  is run transcritical, it is advantageous to regulate the high pressure component of the vapor compression system  10 . By regulating the high pressure of the system  10 , the capacity and/or efficiency of the system  10  can be controlled and optimized. Increasing the gas cooler  14  pressure lowers the enthalpy entering the evaporator  18  and increases capacity, but also requires more energy because the compressor  16  must work harder. By regulating the high pressure of the system  10 , the optimal pressure of the system  10 , which changes as the operating conditions change, can be selected. 
     In a cycle of a prior art vapor compression system  10  illustrated in FIG. 1, the refrigerant exits the compressor  12  at high pressure and enthalpy, shown by point A in FIG.  2 . As the refrigerant flows through the gas cooler  14  at high pressure, it loses heat and enthalpy, exiting the gas cooler  14  with low enthalpy and high pressure, indicated as point B. As the refrigerant passes through the expansion device  16 , the pressure of the refrigerant drops, shown by point C. After expansion, the refrigerant passes through the evaporator  18  and exits at a high enthalpy and low pressure, represented by point D. After the refrigerant passes through the compressor  12 , it is again at high pressure and enthalpy, completing the cycle. 
     FIG. 3 illustrates a vapor compression system  10  employing a flash tank  20  in a two stage economized cycle. The refrigerant exiting the gas cooler  14  is passed through a first expansion device  16   a , reducing its pressure. The refrigerant collects in a flash tank  20  as part liquid  24  and part vapor  22 . The structure of the flash tank  20  is known and forms no part of this invention. The flash tank  20  is controlled in an inventive way in the invention of this application. The vapor  22  is drawn at the top of the flash tank  20  and is used to cool refrigerant that exits the first compression device  12   a . The liquid refrigerant  24  collects at the bottom of the flash tank  20  and is again expanded by a second expansion device  16   b  before entering the evaporator  18 . After the refrigerant passes through the evaporator  18 , it is compressed by the first compression device  12   a , the exhaust being cooled by the cool refrigerant vapor discharged  22  from the flash tank  20 . The refrigerant is then compressed again by a second compression device  12   b  before entering the gas cooler  14 . By using the flash tank  20 , the specific enthalpy of the system can be reduced, which increases the capacity of the system  10 . However, the flash tank  20  has no effect on the high pressure in the gas cooler  14 , which would allow for more control over the high pressure of the system  10 . 
     By utilizing multistage compression, the efficiency of the economized system  10  can be increased where there is a large difference between the high and low pressures in a system. As known, a line  23  communicate vapor  22  to the suction part of the compression stage  12   b . This provides cooling, and is known as economized operation. A thermodynamic diagram of both an economized cycle and a noneconomized cycle is illustrated in FIG.  4 . Economization allows for greater more mass flow through the gas cooler  14 , and reduces the specific enthalpy of the refrigerant that enters the evaporator  18 , causing the cycle to have greater cooling capacity. 
     FIG. 5 illustrates a flash tank  20  and expansion valves  26 ,  28  utilized to regulate the high pressure in a transcritical cycle. A first expansion valve  26  regulates the flow of charge into the flash tank  20  and a second expansion valve  28  regulates the flow of charge out of the flash tank  20 . 
     As known, the flow rate of the charge through the first expansion valve  26  and the second expansion valve  28  is a function of the pressure in the system  10  and the diameter of an orifice in the expansion valves  26 ,  28 . The expansion valves  26 ,  28  are actuated by increasing or decreasing the size of the orifice. By opening or increasing the size of the orifice in the expansion valves  26 ,  28 , the flow rate of charge through the expansion valves  26 ,  28  can be increased. In contrast, by closing or decreasing the size of the orifice in the expansion valves  26 ,  28 , the flow rate of charge through the expansion valves  26 ,  28  can be decreased. By controlling the flow rate of charge though the expansion valves  26 ,  28 , the amount of charge in the flash tank  20 , and the gas cooler  14 , can be regulated to control the pressure in the gas cooler  14 . 
     Control  29  monitors the pressure in the cooler  14  and controls expansion valves  26  and  28 . The control  29  may be the main control for cycle  10 . Control  29  is programmed to evaluate the state the cycle  10  and determine a desired pressure in cooler  14 . Once a desired pressure has been determined, the expansion valves  26  and  28  are controlled to regulate the pressure. The factors that would be used to determine the optimum pressure are within the skill of a worker in the art. 
     If the pressure in the gas cooler  14  is above the optimal pressure, a large amount of energy is used to compress the refrigerant. Control  29  actuates the second expansion valve  28  to close and reduce the volume flow of charge out of the flash tank  20 , increasing the amount of charge in the flash tank  20 , decreasing both the amount of charge and the pressure in the gas cooler  14 . Conversely, if the pressure in the gas cooler  14  pressure is below the optimal pressure, the efficiency of the system  10  could be increased. Control  29  closes the first expansion valve  26  to decrease the volume flow of charge into the flash tank  20 , increasing both the amount of charge and the pressure in the gas cooler  14 . 
     The pressure in the gas cooler  14  is monitored by controller  29 . As the pressure in the gas cooler  14  changes, the controller  29  adjusts the actuation of the expansion valves  26 ,  28  so the optimal pressure can be achieved. 
     By selectively controlling the actuation of the first expansion valve  26  and the second expansion valve  28 , the amount of charge stored in the flash tank  20  can be varied, which varies the high pressure component in the system  10  to achieve optimal capacity and/or efficiency. By regulating the high pressure in the gas cooler  14  before expansion, the enthalpy of the refrigerant at the entry of the evaporator can be modified, controlling the capacity and/or efficiency of the system  10 . 
     While the simplest way to visualize the invention control  29  is to close valve  26  to decrease volume in the flash tank  20  and close valve  28  to increase volume, valve  26  can be opened to increase flow and valve  28  can be opened to decrease volume. 
     As shown in FIG. 6, a third valve  30  and a fourth valve  32  can also be employed to vary the charge level in the flash tank  20  and optimize efficiency and/or capacity of the system  10 . The fourth valve  32  controls the flow of charge from the flash tank  20  to the compression device  12 . By closing the fourth valve  32 , the economizer is turned off and the vapor refrigerant  22  exiting the flash tank  20  is blocked from entering the compressor  12 . Closing the fourth valve  32  traps the vapor refrigerant  20  in the flash tank  20 . The third valve  30  acts as a release and opening the third valve  30  allows the flow of charge from the flash tank  20  to the evaporator  18 . By opening the third valve  30 , the vapor refrigerant  22  from the flash tank  20  is allowed to enter the evaporator  18 , creating and escape for the vapor  22 . Alternatively, the fourth valve  32  can be opened to turn on the economizer. By controlling valves  30  and  32 , the economizer can be turned on and off to optimize the efficiency of the system  10 . The actuation of valves  30 ,  32  is also controlled by the controller  29  which monitors the pressure in the gas cooler  14 . 
     Accordingly, the present invention provides a flash tank  20  utilizing expansion valves  26 ,  28  to control the high pressure in a transcritical vapor compression system  10 . 
     The foregoing description is only exemplary of the principles of the invention. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, so that one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specially described. For that reason the following claims should be studied to determine the true scope and content of this invention.