Patent Publication Number: US-2005126190-A1

Title: Loss of refrigerant charge and expansion valve malfunction detection

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention generally relates to air conditioning and refrigeration systems. More particularly, this invention relates to detecting a loss of refrigerant charge within an air conditioning or refrigeration system. Furthermore, this invention can also be employed for identifying malfunctioning of the expansion valve.  
      2. Description of the Related Art  
      Air conditioning and refrigeration systems need certain refrigerant charge within the system, to achieve a desired amount of cooling within a building, for example. If the refrigerant charge is reduced below a certain level, damage to the system components, such as the compressor, is likely.  
      Typical causes of inadequate refrigerant charge amounts include insufficient charge at the factory or during installation in the field or leakage through damaged components or loose connections.  
      It is necessary to detect a loss of refrigerant charge as early as possible to avoid interrupting system operation, especially during high ambient temperature conditions, when adequate cooling at full-load operation is essential to end users. It is also prudent and critical to diagnose a malfunctioning expansion valve as early as possible to avoid system component damage.  
      While proposals have been made for detecting a loss of refrigerant charge, they are not universally applicable. Further, known arrangements do not provide an early enough indication or are not reliable enough because they can be mistaken for some other system malfunctions such as an evaporator airflow blockage, compressor damage or a plugged distributor. Using known techniques and trying to differentiate between such failure modes requires exhaustive troubleshooting. Furthermore, other consequences of the refrigerant charge loss, such as detection of low suction pressure (i.e., by tripping on a low-pressure switch), usually occur late in the process and applying them may not prevent compressor damage.  
      In addition, the need for detecting refrigerant charge loss becomes especially acute with the introduction of systems that utilize high pressure refrigerants as R410A and R744. Systems with these refrigerants are more prone to leaks.  
      Furthermore, expansion valves in refrigerant systems may malfunction (for example, due to contamination). This in turn may lead to improper system operation and other component damage. Timely detection of such problems is useful to prevent extensive damage and to reduce maintenance.  
      This invention provides a unique early detection of refrigerant charge loss or expansion valve malfunction in the system. The disclosed techniques are useful to prevent compressor damage and to avoid prolonged shutdowns and expensive repairs.  
     SUMMARY OF THE INVENTION  
      This invention utilizes information regarding a superheat value within a refrigerant system for monitoring an amount of refrigerant charge in the system.  
      One method includes determining a refrigerant superheat value within the refrigerant system. By determining a difference between the measured superheat value and an expected superheat value and comparing that difference to a selected threshold, a loss of refrigerant charge can be monitored.  
      One example method includes determining the superheat value based on an actual operating vapor temperature and a saturated vapor temperature. The difference between the saturated vapor temperature and the actual operating vapor temperature is the superheat value.  
      In one example, the method includes determining a superheat value of refrigerant between the compressor and evaporator coil. In another example, the refrigerant system includes an economizer heat exchanger and an evaporator heat exchanger. In this example, the method includes determining superheat value of the refrigerant between the compressor and the evaporator coil or between the compressor and the economizer heat exchanger.  
      In another example, a discharge temperature of refrigerant exiting the compressor is determined to provide a confirmation check on the determined superheat value(s). Using known relationships between the superheat value(s) and the discharge temperature provides the ability to verify the superheat information and, therefore, to determine if refrigerant loss of charge occurs within the system. Similar procedures and techniques are useful to identify a malfunctioning expansion valve.  
      The various features and advantages of this 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  schematically illustrates a refrigerant system designed according to an embodiment of this invention.  
       FIG. 2  schematically illustrates another refrigerant system designed according to another embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       FIG. 1  schematically shows a refrigerant system  20  that may be used as an air conditioning or a refrigeration system. In a cooling mode, a compressor  22  draws refrigerant into a suction port  24  at low pressure and provides a compressed gas into a conduit  28  out of a discharge port  26 . The high temperature, pressurized gas flows through the conduit  28  to a condenser  30  where the gas dissipates heat and usually condenses into a liquid as known. The liquid refrigerant flows through a conduit  32  to an expansion device  34 .  
      The expansion device  34  operates in a known manner to allow the liquid refrigerant to expand and flow into a conduit  36  in the form of a cold, low pressure refrigerant. This refrigerant then flows through an evaporator  38  where the refrigerant absorbs heat from air that flows across the evaporator coil. Subsequently, cool air cools the desired space as known. The refrigerant exiting the evaporator  38  flows through a conduit  40  to the suction port  24  of the compressor  22  where the cycle continues. In one example, the system  20  may also be used as a heat pump where the just-described flow is reversed as known. Some example systems operate in both modes as known and can be utilized as well.  
      In the example of  FIG. 1 , sensors  42 ,  44  and  46  provide information to a controller  50  regarding superheat values within the system  20  such that the controller  50  is capable of making a determination regarding the amount of refrigerant within the system. The amount of superheat is set at a constant (or near constant) value by the expansion valve(s)  34 . When the loss of charge occurs, the expansion valve opens fully to compensate for loss of charge to allow more refrigerant to go through. After enough refrigerant is lost, the expansion valve cannot open any farther to maintain the required superheat. If this occurrence can be detected, then appropriate corrective actions can be taken to fix the problem prior to compressor/system extensive damage.  
      The embodiment of  FIG. 1  includes a temperature sensor  42 , such as a known transducer and a pressure sensor  44 , such as a known transducer, located either within the conduit  40  between the evaporator  38  and the suction port  24  of the compressor  22  or within the evaporator coil  38 . Accordingly, the controller  50  receives temperature and pressure information regarding the refrigerant in the low pressure side of the system and more particularly, the refrigerant that is entering the compressor  22  or leaving the evaporator coil  38  or anywhere in between of these two locations.  
      The controller  50  determines the amount of superheat by subtracting a saturated vapor temperature from the actual operating vapor temperature, which is the temperature of the refrigerant normally determined in the line located between the compressor entrance and exit from the evaporator heat exchanger. The actual operating vapor temperature in  FIG. 1  is provided to the controller  50  by the temperature sensor  42 , which is placed downstream of the evaporator heat exchanger  38 . In this example, instead of using pressure sensor  44 , the saturated vapor temperature is determined from the temperature sensor  46  placed inside the evaporator heat exchanger, preferably in the mid-section of the evaporator coil, in one example.  
      The refrigerant system will normally operate within an acceptable superheat level or range of levels. The controller  50  in this example is programmed to determine a difference between the determined superheat (i.e., based upon the difference between the saturated vapor temperature and the actual operating vapor temperature) and the expected superheat level. When the difference exceeds a selected threshold, the controller  50  determines that the amount of refrigerant within the system is too low.  
      In another example, the controller monitors the superheat level over time to determine changes in the superheat value. In this embodiment, the controller  50  uses known or predicted temperature patterns and is capable of determining when the superheat value begins increasing as a result of the expansion device  34  not being able to open any further to maintain the required superheat levels. The example arrangements are capable of providing an early indication of low refrigerant amount such that appropriate corrective action can be taken to avoid any potential compressor and system damage.  
       FIG. 2  illustrates another example embodiment of a refrigerant system  20 ′ that has a controller  50  that determines the superheat level within the system for purposes of detecting loss of refrigerant charge within the system. This example system operates similar to that of the embodiment of  FIG. 1  with the addition of an economizer heat exchanger  60  downstream of the condenser  30  and upstream of the expansion device  34 . Economizer heat exchangers are generally known. In this example, main refrigerant flow passes through the economizer heat exchanger  60  and the conduit  32 , after the condenser  30 . Another conduit  62  includes an expansion device  64  and is coupled with the economizer heat exchanger  60 . The refrigerant flowing through the conduit  62  and the economizer heat exchanger effectively absorbs heat from refrigerant flowing through the main conduit  32  before that refrigerant reaches the expansion device  34 . Accordingly, the economizer heat exchanger  60  provides further cooling of the main refrigerant flow prior to it reaching the expansion device  34 .  
      A conduit  66  carries refrigerant from the economizer heat exchanger  60  to another inlet economizer port  68  of the compressor  22  at some intermediate pressure. In this example, a pressure sensor  72  and a temperature sensor  74  are associated with the conduit  66  to provide pressure and temperature information to the controller  50  regarding the refrigerant entering the compressor economizer port  68 .  
      The superheat value of refrigerant in the section between the economizer heat exchanger  60  and the economizer port  68  of the compressor  22  is determined using sensors  70 ,  72  and  74  in a fashion similar to the way sensors  42 ,  44  and  46  are applied in the embodiment of this invention shown in  FIG. 1 .  
      Like the embodiment of  FIG. 1 , the controller  50  determines the superheat value in the system  20 ′ and compares that to an expected superheat value. When a difference between the determined superheat and the expected superheat exceeds a selected threshold, the controller  50  determines that the amount of refrigerant in the system is too low.  
      Given this description, those skilled in the art will be able to determine how to select an appropriate threshold for a particular system arrangement and a particular refrigerant used in that system.  
      The inventive arrangement not only provides an indication of potentially reduced refrigerant amount, but also provides the ability to determine if the expansion device  34  or  64  is malfunctioning. As noted above, when the superheat is increasing above a predetermined value, that is an indication that the expansion device cannot open any further to maintain the expected superheat level. It is possible under some circumstances for the expansion device  34  or  64  to be malfunctioning and not opening wide enough to accommodate the desired condition. Accordingly, the determination made by the controller  50  provides an indication of a potential expansion device malfunction.  
      When the controller  50  determines that the superheat value is outside of the expected range, in one example, the controller provides a visual indication on a display screen. In another example, the controller provides an audible alarm or audible signal regarding the determination that the refrigerant amount is too low.  
      In another example, the controller  50  automatically shuts down the system and provides the indication regarding the reason for the shutdown.  
      In the embodiments of  FIG. 1  and  FIG. 2 , the controller  50  can use an additional check on the refrigerant amount within the system by determining a discharge temperature associated with the compressor  22 . When the system is operating properly, the expected discharge temperature can be determined based upon information from the sensors  42 ,  44 ,  72  and  74  regarding pressure and temperature of refrigerant entering the compressor and discharge pressure sensor  76 , for instance. The compressor discharge temperature also can be determined by the controller  50  using known techniques. The compressor discharge temperature is a function of the pressure and temperature entering the compressor and the discharge pressure of the compressor. If the vapor temperature entering the compressor exceeds the preset superheat value, this will result in an increase in discharge temperature above the value that was expected if the entering superheat was within the preset limits. Accordingly, determining any difference between the expected and actual value of the discharge temperature provides a confirmation of the superheat information determined by the controller  50 .  
      It should be noted the previous description would apply to a case of multiple evaporator heat exchangers, multiple economizer heat exchangers or both. In this case the refrigerant superheat can be analyzed independently for each evaporator or economizer heat exchanger section to determine if there is a refrigerant charge loss or malfunctioning expansion valve.  
      The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this invention. The scope of legal protection given to this invention can only be determined by studying the following claims.