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System and method for level control in a flash tankRelated Patent Categories: Refrigeration, Automatic Control, Refrigeration Producer, Of Or By Evaporation ZoneSystem and method for level control in a flash tank description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070151269, System and method for level control in a flash tank. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This patent application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/755,222, filed Dec. 30, 2005. BACKGROUND OF THE INVENTION [0002] The present invention relates generally to controlling an economizer circuit in a chiller system. More specifically, the present invention relates to controlling the level of liquid refrigerant in a flash tank of an economizer circuit. [0003] In refrigeration and chiller systems, a refrigerant gas is compressed by a compressor and then delivered to the condenser. The refrigerant vapor delivered to the condenser enters into a heat exchange relationship with a fluid, e.g., air or water, and undergoes a phase change to a refrigerant liquid. The liquid refrigerant from the condenser flows through corresponding expansion devices to an evaporator. The liquid refrigerant in the evaporator enters into a heat exchange relationship with another fluid, e.g. air, water or other secondary liquid, and undergoes a phase change to a refrigerant vapor. The other fluid flowing through the evaporator is chilled or cooled as a result of the heat-exchange relationship with the liquid refrigerant and is then typically provided to an enclosed space to cool the enclosed space. Finally, the vapor refrigerant in the evaporator returns to the compressor to complete the cycle. [0004] To provide increased capacity, efficiency and performance of the refrigeration or chiller system, an economizer circuit can be incorporated into the system. An economizer circuit can typically include an economizer heat exchanger or flash tank, an inlet line to the flash tank that is connected to the condenser or to a refrigerant circuit downstream of the condenser, an economizer expansion device, which is incorporated in the inlet line, and an outlet line from the flash tank that is connected to a port within the compression chamber of the compressor or to the suction inlet of the compressor. [0005] In flash tank economizer circuits, liquid refrigerant from the condenser flows through the inlet line and expansion device into the flash tank. Upon passing through the expansion device, the liquid refrigerant experiences a pressure drop, whereupon, at least a portion of the refrigerant rapidly expands or "flashes" and is converted from a liquid to a gas. The liquid refrigerant in the flash tank collects at the bottom of the flash tank and returns to the refrigerant circuit through the first outlet line to be provided to the evaporator. The first outlet line may incorporate one or more valves to control the amount of liquid refrigerant returned to the refrigerant circuit. The gaseous refrigerant in the flash tank collects at the top of the flash tank and returns to the compressor, either the suction inlet or a point in the compression chamber at an intermediate pressure, through a second outlet line. The second outlet line, when connected to the compression chamber, may also incorporate one or more valves to control the amount of gaseous refrigerant provided to the compressor. [0006] As discussed above, an economizer circuit can be used to provide increased capacity, efficiency and performance of the refrigeration or chiller system. For example, the economizer circuit can improve system efficiency by providing refrigerant gas at an intermediate pressure to the compressor, thereby reducing the amount of work required by the compressor and increasing compressor efficiency. A variety of parameters in the economizer circuit can be controlled to provide the increased capacity, efficiency and performance of the refrigeration or chiller system. In particular, the amounts of refrigerant entering and leaving the flash tank and the location of the port in the compressor and associated intermediate pressure provided thereto can be controlled or selected, as well as the amount of liquid refrigerant in the tank, to obtain the desired capacity, efficiency and performance of the refrigeration or chiller system. In addition, the economizer circuit can be engaged and disengaged in response to predetermined parameters to further enhance operation of the refrigeration or chiller system. [0007] When the amount of liquid or the level of liquid in the flash tank is used to control the economizer circuit, the liquid level of refrigerant has to be determined. The refrigerant liquid level in the flash tank is usually determined with a sensor or a mechanical device such as float. The control process then usually adjusts system parameters in order to maintain the desired refrigerant liquid level in the flash tank. One disadvantage of this technique is that the sensor or mechanical device could fail thereby preventing efficient operation of the economizer circuit and system. [0008] Therefore, what is needed is a system and method for simply and easily controlling the level of liquid refrigerant in a flash tank of an economizer circuit to provide improved performance to a refrigeration or chiller system. SUMMARY OF THE INVENTION [0009] One embodiment of the present invention is directed to a method for controlling an economizer circuit in a chiller system. The method includes the steps of providing an economizer circuit for a chiller system having a flash tank, an inlet line, and a feed valve. The feed valve is located in the inlet line. The feed valve is configured to control flow of refrigerant to the flash tank. The method further includes measuring at least one system operating parameter for the chiller system; calculating a valve position for the feed valve in response to the measured at least one system operating parameter; and adjusting the feed valve to the calculated valve position to control the level of liquid refrigerant in the flash tank. [0010] Another embodiment of the present invention is directed to a liquid level control system for an economizer circuit of a chiller system. The system includes a flash tank, an inlet line, and a feed valve. The feed valve is located in the inlet line and configured to control flow of refrigerant to the flash tank. The liquid level control system includes a map of a plurality of operating positions of the feed valve. Each of the plurality of operating position is associated with a predetermined position of the feed valve and an amount of refrigerant in the flash tank. The position of the feed valve and the amount of refrigerant in the flash tank correspond to a flow rate of the predetermined feed valve position. The map is configured to correlate the plurality of feed valve operating positions with a plurality of predetermined system operating parameters. [0011] The system also includes a microprocessor. The microprocessor is configured to control the position of the feed valve to control the level of liquid refrigerant in the flash tank. The microprocessor generates a control signal to position the adjustable valve arrangement of the economizer circuit based on the map the to control the operation of the feed valve. [0012] Another embodiment of the present invention is directed to a chiller system. The chiller system includes a refrigerant circuit with a compressor, a condenser arrangement, an expansion valve and an evaporator arrangement connected in a closed refrigerant loop. An economizer circuit is connected to the refrigerant circuit. The economizer circuit includes a flash tank, an inlet line, and a feed valve. The feed valve is disposed in the inlet line and configured to control flow of refrigerant to the flash tank. Also, a control panel for the chiller system includes a map of a plurality of operating positions of the feed valve. Each operating position of the plurality of operating positions is associated with a predetermined position of the feed valve and an amount of refrigerant in the flash tank corresponding to a flow rate of the predetermined position. The map is configured to correlate the plurality of feed valve operating positions with a plurality of predetermined system operating parameters. A microprocessor in the control panel is configured to control the position of the feed valve to control the level of liquid refrigerant in the flash tank. The microprocessor generates a control signal to position the feed valve based on the map. [0013] One advantage of the present invention is that float valves or electronic level sensors are not required in the flash tank, thereby lowering the cost and complexity of the system while increasing the reliability of the system. [0014] Another advantage of the present invention is that operation of the economizer circuit can be finely tuned to obtain a desired level in the flash tank by positioning the feed valve in response to system conditions. [0015] Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS [0016] FIG. 1 illustrates an embodiment of a refrigeration or chiller system used with the present invention. [0017] FIG. 2 is a flowchart showing an embodiment of the economizer feed valve control process of the present invention. [0018] Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION OF THE INVENTION [0019] FIG. 1 illustrates generally an application that can be used with the present invention. As shown in FIG. 1, the HVAC, refrigeration or liquid chiller system 100 includes a compressor 102, a condenser arrangement 104, expansion device(s) 105, a liquid chiller or evaporator arrangement 106 and the control panel 108. The compressor 102 can be driven by a motor 124 that is powered by a variable speed drive (VSD) 122. In addition, the chiller system 100 has an economizer circuit that includes an economizer heat exchanger or flash tank 110, an inlet line 112, an economizer feed valve 114, a first outlet line 116, and a second outlet line 118. Continue reading about System and method for level control in a flash tank... 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