Refrigerant flow rate control based on evaporator dryness
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 08/205,858, filed Mar. 3, 1994 and assigned to the same assignee as the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to refrigerant expansion control in refrigeration systems and more particularly concerns using pulse width modulated solenoid valves for expansion control. As used herein, the term &#34;refrigeration system&#34; refers to refrigerators, air conditioners or any other system which produces a refrigeration effect. 
     Conventional refrigeration systems used in household refrigerators typically operate on the simple vapor compression cycle. Such a cycle includes a compressor, a condenser, an expansion device, and an evaporator all connected in series in the order given and charged with a refrigerant. The refrigerant is compressed by the compressor to high temperature and pressure and then condensed by the condenser where it loses heat to the ambient. The liquid refrigerant next flows through an expansion device, such as an expansion valve or a capillary tube, so that it undergoes adiabatic expansion. The now low pressure refrigerant flows through the evaporator and is vaporized by absorbing heat from air passing over the evaporator. The cooled air is used to refrigerate one or more refrigerator compartments. The gaseous or mostly gaseous refrigerant exiting the evaporator is returned to the compressor via a suction line to repeat the cycle. 
     Household refrigerators typically use a capillary tube to control refrigerant expansion because it is a simple, low cost device. However, capillary tubes have a number of limitations as expansion devices. For instance, capillary tubes must be made very long to allow an inside diameter which is manufacturable and large enough to avoid clogging. This needed length takes up space in the refrigerator. The use of capillary tube expansion control also requires very precise refrigerant charging operations during production because the flow rate through the capillary tube is highly sensitive to the amount of refrigerant charge in the system. 
     Furthermore, a capillary tube can be sized to provide the optimum refrigerant flow rate for only one operating condition. Capillary tubes are thus typically sized to provide the optimum flow rate for normal operating conditions. This means that when the refrigeration cycle begins (as well as under high load conditions), the capillary tube is undersized, and the evaporator is starved of refrigerant. This reduces the cooling capacity and efficiency of the refrigerator. Near the end of the refrigeration cycle, the capillary tube will be oversized and the evaporator will be flooded, again reducing efficiency. Because of this, cycle efficiency using capillary tube expansion is considerably below that attainable with active expansion control. 
     However, active expansion control, in the form of conventional thermostatic expansion valves, does not work well in household refrigerators. While thermostatic expansion valves are often used in automotive air conditioning and commercial refrigeration systems which have large refrigerant flow rates, they cannot be made with orifices small enough to regulate the very low flow rates (typically 10-12 lb/hr) of household refrigerators. That is, to achieve the required pressure drop the valve orifice would need to be on the order of 10 mils or less, a size that is impractical to manufacture and very susceptible to plugging. 
     Accordingly, there is a need for an alternative to capillary tubes and thermostatic expansion valves as expansion control in household refrigerators. 
     SUMMARY OF THE INVENTION 
     The above-mentioned needs are met by the present invention which uses a pulse width modulated solenoid valve for expansion control. A pulse width modulated control signal is generated for cyclically opening and closing the valve. The pulse width of the control signal determines the average flow rate through the valve. The duty cycle of the valve is varied in accordance with evaporator dryness to precisely control the average flow rate. 
     Specifically, the present invention provides a refrigeration system comprising an evaporator disposed in the compartment to be cooled. A pulse width modulated solenoid valve is connected to the inlet of the evaporator by an evaporator inlet line. A first temperature sensor is located near the evaporator inlet, and a second temperature sensor is disposed in the compartment. A controller is included for controlling the duty cycle of the pulse width modulated solenoid valve. The controller receives inputs from the two temperature sensors and controls the valve duty cycle based on the difference of the two temperatures. The first temperature sensor can be located on either the evaporator or the evaporator inlet line. The second temperature sensor is preferably located on the back wall of the compartment. Possible locations on the back wall include: adjacent to the air inlet to the compartment, adjacent to the air outlet to the compartment, and about two inches below the air outlet. 
     Measuring the difference between the compartment air temperature and the evaporator inlet temperature permits accurate, simple control and reduces the tendency of &#34;hunting.&#34; Since the evaporator inlet temperature is the first to change when the valve is adjusted, rapid feedback is obtained. The compartment air temperature represents a reference point for the maximum possible evaporator exit temperature and provides a basis for a very stable control signal. 
     Several advantages are also realized by using a pulse width modulated solenoid valve for expansion control. Because of its oscillating manner of operating, the pulse width modulated solenoid valve can be made with a larger orifice, thereby avoiding plugging problems. Systems using pulse width modulated control are relatively insensitive to total refrigerant charge which eases charging requirements during production. Pulse width modulated solenoid valves are beneficial for use with variable and/or multi-speed compressors because of the ability to match different flow rates as the compressor displacement changes. During the off cycle, a pulse width modulation solenoid valve can be used to maintain a positive seal between the high and low pressures, thereby preventing refrigerant migration and conserving energy. The solenoid valve therefore acts as an energy valve, eliminating the need for a separate valve to serve this function. 
     Other objects and advantages of the present invention will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
     FIG. 1 is a schematic representation of one embodiment of a refrigeration cycle in accordance with the present invention; and 
     FIG. 2 is an illustration of a pulse width modulated frequency signal used in accordance with the present invention to control an expansion valve. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 shows a refrigeration system 10 comprising a compressor 12, a condenser 14, an expansion valve 16, and an evaporator coil 18, all connected in a closed series loop in the order given. The refrigeration system 10 is charged with a refrigerant which is compressed in the compressor 12. The compressed refrigerant is then discharged to the condenser 14, where it is cooled and condensed and expelled into a liquid line 20. The liquid refrigerant then flows through the expansion valve 16, expanding while it does so. The refrigerant flows from the expansion valve 16 to the evaporator 18 via an evaporator inlet line 21. The evaporator 18 is preferably disposed within a compartment 22. 
     As the refrigerant flows through the evaporator 18, it is in a heat exchange relationship with the air in the compartment 22. Thus, heat is transferred from the compartment 22 to the refrigerant flowing through the evaporator 18, causing the liquid refrigerant to evaporate. The refrigerant preferably assumes a slightly superheated gaseous state by the time it exits the evaporator 18. The gaseous refrigerant is then passed through a suction line 24 and returned to the compressor 12, where the cycle is repeated. Although not shown in FIG. 1, it should be understood that the liquid line 20 and the suction line 24 can be arranged in a countercurrent heat exchange relationship so as to improve cycle efficiency. This heat transfer relationship is typically accomplished by soldering the respective lines (or at least portions thereof) together. 
     Although the refrigeration system 10 is described herein as operating on the simple vapor compression cycle, the present invention is equally applicable to other refrigeration cycles. For instance, the present invention could be used in a refrigeration system operating on the dual evaporator cycle described in U.S. Pat. No. 4,910,972, issued Mar. 27, 1990 to Heinz Jaster and herein incorporated by reference. 
     The expansion valve 16 is the throttling or metering device which controls the operation of the refrigeration system 10. In accordance with the present invention, the expansion valve 16 is a pulse width modulated solenoid valve which is controlled by a controller 26 as part of a feedback loop. The controller 26 controls the expansion valve 16 on the basis of the evaporator dryness. The level of dryness (i.e., the amount of liquid refrigerant) indicates whether the evaporator 18 requires more refrigerant. In vapor compression cycle refrigeration systems, the difference between the air temperature in the refrigerated compartment 22 and the evaporator inlet temperature provides an indication of the evaporator dryness. Generally, as this temperature difference increases, the valve 16 should be opened more to increase the refrigerant flow rate. 
     In the present invention, the compartment air temperature is measured by an air temperature sensor 28 located in the compartment 22. Such a sensor may already be provided in the refrigerator to control operation of the compressor 12. The evaporator inlet temperature is measured by an inlet temperature sensor 30 located at or near the inlet of the evaporator 18. The temperature sensors 28, 30 are preferably solid state sensors but any suitable sensors, such as resistance temperature detectors (RTDs), thermocouples or thermistors, can be used. 
     The controller 26 receives signals corresponding to the compartment air and evaporator inlet temperatures from the air temperature sensor 28 and the inlet temperature sensor 30, respectively. Based on these temperature signals, the controller 26 produces a control signal 32 which is fed to the solenoid valve 16. The control signal 32 is a pulse width modulated frequency signal which causes the solenoid valve 16 to oscillate between a fully open condition and a fully closed condition such that the duty cycle of the open-to-closed conditions determines the average flow rate through the expansion valve 16. The pulse width is adjusted in accordance with the detected evaporator dryness to maintain dryness at a relatively fixed level. Preferably, the difference between the air temperature in the compartment 22 and the evaporator inlet temperature is held in the range of about 5°-15° F. By controlling the duty cycle of the valve 16 to maintain the desired evaporator dryness, optimal system performance can be obtained. 
     FIG. 2 shows a sample waveform for the control signal 32. The waveform is a square wave which alternates between a maximum control voltage V 1  and a minimum control voltage V 2 . When the waveform is in the maximum control voltage V 1 , the valve 16 is moved to the fully open condition, and when the waveform is in the minimum control voltage V 2 , the valve 16 is moved to the fully closed condition. Instead of an instantaneous voltage change, the pulse width modulated waveform shown in FIG. 2 has a brief transition period between the maximum and minimum voltages. This avoids the problem of a pressure shockwave being generated in the refrigerant which can occur when an expansion valve is abruptly opened and closed. The frequency of the waveform is constant regardless of system flow rate demand conditions. Preferably, this frequency is set in the range of about 0.1-2 hertz. 
     The average flow rate through the expansion valve 16 is dependent on the duty cycle of the pulse width modulated waveform. Thus, prior to the time t 0  shown in FIG. 2, the time the valve 16 is fully open is less than the time the valve 16 is fully closed, thereby producing a relatively low average flow rate. After time t 0  (at which time increased dryness is detected indicating a higher demand for refrigerant), the controller 26 adjusts the duty cycle so that the time the valve 16 is fully open is increased with respect to the time the valve 16 is fully closed, thereby producing a larger average flow rate. 
     Preferably, the expansion valve 16 is a normally closed valve; that is, the valve 16 closes when the solenoid coil is not energized. This means that the minimum control voltage V 2  can be zero for valve closure. Furthermore, power to the valve 16 is interrupted whenever the compressor 12 is shut down. The valve 16 thus remains closed during periods of compressor shut-down. This prevents refrigerant migration to the evaporator 18 during the off cycle, thereby conserving energy. The solenoid valve 16 therefore acts as an energy valve, eliminating the need for a separate valve to serve this function. 
     The controller 26 can comprise one of a variety of pulse width modulation control schemes known in the art. One suitable control scheme is described in the copending application entitled &#34;Control Circuit for Refrigerant Metering Solenoid Valve,&#34; Ser. No. 08/222,051, filed concurrently herewith. This copending application, which is assigned to the same assignee as the present invention, is herein incorporated by reference. Other suitable pulse width modulation controllers are described in U.S. Pat. No. 4,651,535 issued Mar. 24, 1987 to Richard H. Alsenz and in U.S. Pat. No. 5,255,530 issued Oct. 26, 1993 to Donald E. Janke, both of which are herein incorporated by reference. 
     The exact location of the air temperature sensor 28 within the compartment 22 has a large impact on the response of the control circuit 26, particularly when the refrigeration system 10 is part of a household refrigerator; in which case, the compartment 22 is typically the freezer compartment of the refrigerator. When used in refrigerators, the air temperature sensor 28 will generally be located in the rear of the freezer compartment. This not only facilitates construction, but also shields the sensor 28 from temperature stratifications caused by opening the freezer door. 
     One specific location for the air temperature sensor 28 is adjacent to the freezer air inlet which is typically at the bottom of the freezer compartment. The sensor 28 is exposed to the coldest air temperatures here and thus provides a more direct indication of when the valve 16 needs to be adjusted. However, this location remains very cold during the off cycle and thus tends to give false temperature signals so that start up response time is not as fast. False temperature signals due to door openings are also exacerbated. Another possible location is adjacent to the freezer air outlet at the top of the freezer compartment. Here, the air temperatures are generally the warmest and start up response time is the quickest and false signals are minimized, although valve control may suffer. 
     The air temperature sensor 28 can also be located anywhere between the air inlet and the air outlet to balance the respective benefits of these two locations. One preferred location for the air temperature sensor 28 is on the back wall of the freezer compartment, about two inches below the air outlet. 
     The inlet temperature sensor 30 is located close enough to the inlet of the evaporator 18 so that evaporator inlet temperature can be measured. The inlet temperature sensor 30 is preferably positioned inside of the compartment 22, although it may be located outside. The exact location of the inlet temperature sensor 30 will affect the start up response time of the cycle. At start up, liquid refrigerant is initially drawn out of the evaporator 18 by the compressor 12, resulting in a false evaporator inlet temperature signal. Thus, the quicker the inlet temperature sensor 30 is re-exposed to refrigerant, the quicker the refrigeration system 10 will reach its normal operating condition. For this reason, it is expedient to locate the inlet temperature sensor 30 on the evaporator inlet line 21 as near as possible to the expansion valve 16 while still accurately measuring evaporator inlet temperature. However, during the off cycle, ambient heat may be conducted to the inlet temperature sensor 30 via the evaporator inlet line 21. If the heat conduction is substantial enough to cause false temperature signals, then the inlet temperature sensor 30 may be placed downstream from the valve 16. The inlet temperature sensor 30 could even be placed on the evaporator 18, near the inlet, to avoid excessive heat conduction. 
     The foregoing has described an improved refrigeration system which uses a pulse width modulated solenoid valve for expansion control. The valve is controlled on the basis of evaporator dryness. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.