Title:
System and method for level control in a flash tank
Kind Code:
A1


Abstract:
A control algorithm is provided for controlling an economizer circuit in a chiller system. The control algorithm adjusts the position of a feed valve in the economizer circuit in response to measured system operating parameters to maintain a level of liquid refrigerant in a flash tank of the economizer circuit. The measured system operating parameters can include the load of the compressor and the pressure and temperature of refrigerant in the flash tank.



Inventors:
Crane, Curtis Christian (York, PA, US)
Hill IV, Frank Highland (York, PA, US)
Application Number:
11/428913
Publication Date:
07/05/2007
Filing Date:
07/06/2006
Assignee:
JOHNSON CONTROLS TECHNOLOGY COMPANY (Holland, MI, US)
Primary Class:
Other Classes:
62/222, 62/513, 62/218
International Classes:
F25B41/04
View Patent Images:



Primary Examiner:
CARTON, MICHAEL
Attorney, Agent or Firm:
MCNEES WALLACE & NURICK LLC (HARRISBURG, PA, US)
Claims:
What is claimed is:

1. A method for controlling an economizer circuit in a chiller system, the method comprising 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 being disposed in the inlet line and being configured to control flow of refrigerant to the flash tank; 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.

2. The method of claim 1, wherein the at least one system operating parameters is selected from a load of the compressor, a pressure of refrigerant in the flash tank, or a temperature of refrigerant in the flash tank.

3. The method of claim 2, wherein the step of measuring at least one system operating parameter includes determining the load on the compressor by determining a system capacity from an entering fluid temperature and a leaving fluid temperature in the evaporator

4. The method of claim 2, wherein the step of measuring at least one system operating parameter includes determining the load on the compressor by sensing the flow of liquid through the evaporator

5. The method of claim 2, wherein the step of measuring at least one system operating parameter includes determining the load on the compressor by measuring a speed of the compressor.

6. The method of claim 2, wherein the step of measuring at least one system operating parameter includes determining the load on the compressor by sensing an operating frequency of the variable speed drive.

7. The method of claim 2, wherein the step of measuring at least one system operating parameter includes determining the load on the compressor by sensing the position of a slide valve in the compressor.

8. The method of claim 2, wherein the step of measuring at least one system operating parameter includes determining the pressure of the refrigerant in the flash tank by sensing the pressure in the flash tank

9. The method of claim 2, wherein the step of measuring at least one system operating parameter includes determining the temperature of the refrigerant in the flash tank by sensing the temperature in the flash tank.

10. The method of claim 2, wherein the step of measuring at least one system operating parameter includes determining the pressure of the refrigerant in the flash tank by sensing a suction pressure and a discharge pressure in the compressor.

11. The method of claim 2, wherein the step of measuring at least one system operating parameter includes determining the temperature of the refrigerant in the flash tank by sensing a suction temperature and a discharge temperature of the refrigerant in the compressor.

12. The method of claim 1, wherein the step of calculating a valve position includes comparing the measured at least one system operating parameter to entries in a map to determine the position for the feed valve.

13. The method of claim 12, wherein the map correlates the measured at least one system operating parameter and at least one additional criteria with operating positions for the feed valve.

14. The method of claim 13, wherein the at least one additional criteria is selected from the group consisting of at least one cross-sectional flow area for the feed valve corresponding with at least one operating position of the feed valve, at least one actuation requirement of a motor for adjusting the operating position of the feed valve, and performance of the compressor at a plurality of measured operating parameters based over a range of system operating conditions.

15. The method of claim 14, wherein the ranges of system operating parameters are based on measured or calculated mapping of the compressor.

16. A liquid level control system for an economizer circuit having a feed valve being disposed in an inlet line and configured to control flow of refrigerant to a flash tank, the control system comprising: a map of a plurality of operating positions of the feed valve, each operating position of the plurality of operating positions being 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 configured to correlate the plurality of feed valve operating positions with a plurality of predetermined system operating parameters; a microprocessor, the microprocessor being configured to control the position of the feed valve to control the level of liquid refrigerant in the flash tank; and wherein the microprocessor generates a control signal to position the adjustable valve arrangement of a refrigeration system based on the map the to control the operation of the feed valve.

17. The system of claim 16, wherein the map is generated from test data that determines an operating position for the valve in response to particular system operating parameters or conditions.

18. The system of claim 16, wherein the map is generated from calculated data that determines an operating position for the valve in response to particular system operating parameters or conditions.

19. The system of claim 16, also including at least one upper level switch to determine a refrigerant level of the flash tank above a predetermined maximum refrigerant level, and at least one lower level switch to determine a predetermined minimum refrigerant level, the upper and lower level switches configured to generate a signal to indicate to the microprocessor when the level of exceeds a respective maximum or minimum refrigerant level.

20. The system of claim 16, wherein the predetermined system operating parameters is selected from a load of the compressor, a pressure of the refrigerant in the flash tank, or a temperature of the refrigerant in the flash tank.

21. The system of claim 20, wherein the load on the compressor is determined by determining a system capacity by sensing an entering liquid temperature and a leaving liquid temperature in the evaporator.

22. The system of claim 20, wherein the load on the compressor is determined by at least on of sensing the flow of liquid through the evaporator, measuring a speed of the compressor, sensing an operating frequency of the variable speed drive, or sensing the position of a slide valve in the compressor.

23. The system of claim 20, wherein the pressure of the refrigerant in the flash tank is determined by at least one of sensing the pressure in the flash tank or sensing a suction pressure and a discharge pressure in the compressor.

24. The system of claim 20, wherein the temperature of the refrigerant in the flash tank is determined by at least one of sensing the temperature in the flash tank or sensing a suction temperature and a discharge temperature of the refrigerant in the compressor.

25. A chiller system comprising: 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 connected to the refrigerant circuit, the economizer circuit comprising a flash tank, an inlet line, and a feed valve, the feed valve being disposed in the inlet line and being configured to control flow of refrigerant to the flash tank; and a control panel comprising: a map of a plurality of operating positions of the feed valve, each operating position of the plurality of operating positions being 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 configured to correlate the plurality of feed valve operating positions with a plurality of predetermined system operating parameters; a microprocessor, the microprocessor being configured to control the position of the feed valve to control the level of liquid refrigerant in the flash tank; and wherein the microprocessor generates a control signal to position the feed valve based on the map.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

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

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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

FIG. 1 illustrates an embodiment of a refrigeration or chiller system used with the present invention.

FIG. 2 is a flowchart showing an embodiment of the economizer feed valve control process of the present invention.

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

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.

The VSD 122 receives AC power having a particular fixed line voltage and fixed line frequency from an AC power source and provides AC power to the motor 124 at desired voltages and desired frequencies, both of which can be varied to satisfy particular requirements. Preferably, the VSD 122 can provide AC power to the motor 124 that may have higher voltages and frequencies and lower voltages and frequencies than the rated voltage and frequency of the motor 124. The motor 124 is preferably an induction motor that is capable of being operated at variable speeds. However, any suitable motor that can be operated at variable speeds can be used with the present invention.

Compressor 102, driven by motor 124, compresses a refrigerant vapor and delivers the vapor to the condenser 104 through a discharge line. The compressor 102 is preferably a screw compressor, but can be any suitable type of compressor, e.g., centrifugal compressor, reciprocating compressor, etc. The refrigerant vapor delivered by the compressor 102 to the condenser 104 enters into a heat exchange relationship with a fluid, e.g., air or water, 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 104 flows through an expansion device 105 to an evaporator 106.

The evaporator 106 can include connections for a supply line and a return line of a cooling load. A secondary liquid, e.g. water, ethylene, calcium chloride brine or sodium chloride brine, travels into the evaporator 106 via return line and exits the evaporator 106 via supply line. The liquid refrigerant in the evaporator 106 enters into a heat exchange relationship with the secondary liquid to lower the temperature of the secondary liquid. The refrigerant liquid in the evaporator 106 undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the secondary liquid. The vapor refrigerant in the evaporator 106 exits the evaporator 106 and returns to the compressor 102 by a suction line to complete the cycle. It is to be understood that any suitable configuration of condenser 104 and evaporator 106 can be used in the system 100, provided that the appropriate phase change of the refrigerant in the condenser 104 and evaporator 106 is obtained.

The economizer circuit is incorporated in the main refrigerant circuit between the condenser 104 and the expansion device 105. The economizer circuit has an inlet line 112 that is either connected directly to or is in fluid communication with the condenser 104. The inlet line 112 has an economizer feed valve 114 upstream of the flash tank 110. The economizer feed valve 114 operates to regulate the amount of refrigerant entering the flash tank 110. The refrigerant entering the flash tank 110 is preferably at a pressure below the discharge pressure of the compressor 102 and above the suction pressure of the compressor 102. In a preferred embodiment, the feed valve 114 can also operate as an expansion valve to lower the pressure of the liquid refrigerant from the condenser 104 flowing through the economizer feed valve 114. In another embodiment, one or more expansion valves can be incorporated into the economizer circuit downstream of the feed valve 114 before the flash tank 110. Downstream of the economizer feed valve 114, both liquid refrigerant and gaseous refrigerant enters the flash tank 110. Inside the flash tank 110, gaseous refrigerant preferably collects in the top or upper portion of the flash tank 110 and the liquid refrigerant preferably settles in the bottom or lower portion of the flash tank 110. The flash tank 110 can include one or more upper level switches 140 and one or more lower level switches 142. The level switches 140, 142 can determine when the level of liquid in the flash tank is either above or below the corresponding level switch. The level switches 140, 142 can provide the control panel 108 with a signal indicating whether the level of liquid in the flash tank is above, or alternately, below, the corresponding level switch. Any suitable level switch can be used for level switches 140, 142, but simple, low cost, reliable level switches are preferred.

The liquid refrigerant in the flash tank 110 flows or travels through the first outlet line 116 to the expansion valve 105. The expansion valve 105 can be a thermal expansion valve, an electronic expansion valve, an orifice or any other suitable metering device or valve. The second outlet line 118 preferably returns the gaseous refrigerant in the flash tank 110 to an economizer port in the compressor 102 connected directly to a compression chamber of the compressor 102. Alternatively, second outlet line 118 can return the gaseous refrigerant in the flash tank 110 to the suction inlet of the compressor 102. The second outlet line 118 may include one or more economizer port valves to control the flow of gaseous refrigerant from the flash tank 110 to the compressor 102.

A conventional HVAC, refrigeration or liquid chiller system 100 with an economizer circuit includes many other features that are not shown in FIG. 1. These features have been purposely omitted to simplify the drawing for ease of illustration. Furthermore, while FIG. 1 illustrates the HVAC, refrigeration or liquid chiller system 100 as having one compressor connected in a single refrigerant circuit, it is to be understood that the system 100 can have multiple compressors connected into each of one or more refrigerant circuits. In addition, each refrigerant circuit can have its own economizer circuit(s) as described above.

The control panel 108 can include an analog to digital (A/D) converter, a microprocessor, a non-volatile memory, and an interface board to control operation of the refrigeration system 100. The control panel 108 can also be used to control the operation of the VSD 122, the motor 124 and the compressor 102. The control panel 108 executes a control algorithm(s) or software to control operation of the system 100 and to determine and implement an operating configuration or position for the economizer feed valve 114 to control the level of liquid refrigerant in the flash tank 110. In one embodiment, the control algorithm(s) can be computer programs or software stored in the non-volatile memory of the control panel 108 and can include a series of instructions executable by the microprocessor of the control panel 108. While it is preferred that the control algorithm be embodied in a computer program(s) and executed by the microprocessor, it is to be understood that the control algorithm may be implemented and executed using digital and/or analog hardware by those skilled in the art. If hardware is used to execute the control algorithm, the corresponding configuration of the control panel 108 can be changed to incorporate the necessary components and to remove any components that may no longer be required.

The control panel 108 uses a map, table or database of feed valve operating positions to control the operation of the feed valve 114. The operating position for the feed valve 114 relates to the size of the valve opening and the corresponding amount of refrigerant that flows through the valve opening. The map or table used by the control panel 108 correlates the feed valve operating positions with system operating parameters. The map or table can be generated from test data that determines an operating position for the valve in response to particular system operating parameters or conditions. Preferably, one map may be applied to more than one system, and would be applicable to, for example, a family of products.

FIG. 2 illustrates an embodiment of the economizer feed valve control process of the present invention. The feed valve control process can be initiated in response to a starting command or instruction from a capacity control process or other control program for the system 100. The economizer feed valve control process can be a stand-alone process or program or it can be incorporated into a larger control process or program, such as a capacity control program for the chiller system.

The process begins by measuring system operating parameters in step 202. The measured system operating parameters are preferably the load on the compressor 102 and the pressure and temperature of the refrigerant in the flash tank 110, although additional, fewer or alternate system operating parameters can be measured. The load on the compressor 102 can be measured or determined in several ways including: measuring the system capacity by sensing the entering and leaving liquid temperature in the evaporator 106 or by sensing the flow of liquid through the evaporator 106; measuring the speed of the compressor 102; sensing an operating frequency of the variable speed drive 122; and sensing the position of a slide valve in the compressor 102. The pressure and temperature of the refrigerant in the flash tank 110 can be determined in several ways including sensing the temperature and pressure in the flash tank 110 and by sensing the suction and discharge pressures and/or temperatures in the compressor 102.

In step 204, the measured system operating parameters are compared to entries in the map to determine the appropriate operating position for the feed valve 114. The entries in the map correlate the measured system operating parameters (the loading of the compressor 102 and the temperature and pressure of the refrigerant in the flash tank 110) and other information with operating positions for the feed valve 114. The other information in the map can relate to relationships of operating positions of the feed valve 114 and a corresponding cross-sectional flow area for the feed valve 114, any actuation requirement of a motor, preferably a stepper motor, to adjust the operating position of the feed valve 114, and/or knowledge of the compressor's performance at the measured operating parameters based on measured or calculated mapping of the compressor at a range of conditions representative of the conditions that the system 100 will be operated.

The map can be a look up table having operating positions for the feed valve 114 based on the measured system operating parameters. Alternately, the feed valve position can be computed based on a multiple-variable algorithm, or on graphical curves of one or more variables, wherein the variables can be the measured system operating parameters. Some examples of system operating parameters that can be used as variables include the temperature and pressure in the flash tank 110, the suction and discharge pressures and/or temperatures in the compressor 102, the speed of the compressor 102, and the position of the expansion valve 105 or drain valve. The feed valve 114 can be a digital incremental position type or an analog type. Further, the signal for opening or closing the feed valve 114 corresponds to the type of valve that is used. In one embodiment, the opening and closing of the feed valve 114 is based on the load of the system 100, i.e., the valve has smaller movements during lighter load conditions and larger movements during heavier load conditions. Preferably, the feed valve 114 is controlled to maintain a predetermined level of refrigerant inside of the flash tank 110.

After an operating position for the feed valve 114 is determined or calculated in step 204, the process proceeds to step 206. In step 206, the position of the feed valve 114 is adjusted to the desired or calculated position. After the feed valve 114 is positioned, the control returns to step 202 and the process is repeated. As discussed in more detail below, the rate of change in the feed valve position may be controlled. The system then returns to step 202 and repeats the process. An optional time delay 208 may be built in to prevent hunting or instability in the system. In a preferred embodiment the feed valve 114 is not allowed to be moved from a full open position to a closed position in a single step, except under predefined conditions, such as in response to the upper level switch 140 detecting an overfull condition. However, under normal conditions, the system controls the rate at which the feed valve position may change to avoid the feed valve 114 from being forced fully open or closed in a single step.

In a preferred embodiment, the level switches 140, 142 can be used in conjunction with the control process of FIG. 2 to further regulate the level of liquid refrigerant in the flash tank 110. If the level switches 140, 142 detect that the liquid level in the flash tank 110 is above an upper level limit or that the liquid level in the flash tank 110 is below a lower level limit, the control panel 108 can alert an operator of the system 100 of the liquid level or the control system can take further action separate from the control process of FIG. 2 to remedy the situation. The level switches 140, 142 can provide an immediate control system signal, to open the feed valve 114 in response to a low refrigerant level in the flash tank 110, or to close the feed valve 114 in response to a high refrigerant level in the flash tank 110, until the desired refrigerant level is obtained.

In one embodiment of the present invention, the frequency and duration of excursions or deviations from between the upper and lower level limits can be used to adjust the operating map. Fuzzy logic reasoning, or other suitable techniques, can be used to update the operating map to attempt to avoid the deviations from between the upper and lower level limits. Further adjustments or offsets in the operating map or algorithm can be made based on the particular chiller system having different operating characteristics.

In one embodiment, the system implements a lookup table, which is preferably a multi-dimensional lookup table. Also, the map or lookup table may adaptively adjust based on feedback from the upper and lower limit switches 140, 142. The upper and lower limit switches 140, 142 may also be used to offset the desired position for the feed valve 114 and adjust the map parameters accordingly. For example, if the refrigerant level in the tank detects only high level indications, the map parameters would be offset downwards so that the normal position of the feed valve 114 is less open, and if only low levels are detected, the map parameters would be offset accordingly, so that the position of the feed valve 114 is normally in a more open position.

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.