Abstract:
A method and apparatus for determining the sufficiency of refrigerant charge in an air conditioning system using a single temperature sensor for sensing three different temperatures within the system to compute a condenser approach temperature difference, which in then compared with a predetermined optimal condenser approach temperature difference to indicate the charge condition of the system. The device includes an absorbent pad for sensing wet bulb temperatures, and is formed as a clamshell that can be clamped onto the condenser liquid line. A microprocessor is included to make the comparison and to appropriately display the result as a visual indication of charge adequacy.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to air conditioning systems and, more particularly, to a method and apparatus for determining proper refrigerant charge in such systems.  
         [0002]     Maintaining proper refrigerant charge level is essential to the safe and efficient operation of an air conditioning system. Improper charge level, either in deficit or in excess, can cause premature compressor failure. An over-charge in the system results in compressor flooding, which, in turn, may be damaging to the motor and mechanical components. Inadequate refrigerant charge can lead to increased power consumption, thus reducing system capacity and efficiency. Low charge also causes an increase in refrigerant temperature entering the compressor, which may cause thermal over-load of the compressor. Thermal over-load of the compressor can cause degradation of the motor winding insulation, thereby bringing about premature motor failure.  
         [0003]     Charge adequacy has traditionally been checked using either the “superheat method” or “subcool method”. For air conditioning systems which use a thermal expansion valve (TXV), or an electronic expansion valve (EXV), the superheat of the refrigerant entering the compressor is normally regulated at a fixed value, while the amount of subcooling of the refrigerant exiting the condenser varies. Consequently, the amount of subcooling is used as an indicator for charge level. Manufacturers often specify a range of subcool values for a properly charged air conditioner. For example, a subcool temperature range between 10 and 15° F. is generally regarded as acceptable in residential cooling equipment. For air conditioning systems that use fixed orifice expansion devices instead of TXVs (or EXVs), the performance of the air conditioner is much more sensitive to refrigerant charge level. Therefore, superheat is often used as an indicator for charge in these types of systems. A manual procedure specified by the manufacturer is used to help the installer to determine the actual charge based on either the superheat or subcooling measurement. Table 1 summarizes the measurements required for assessing the proper amount of refrigerant charge.  
                                           TABLE 1                           Measurements Required for Charge Level Determination                Superheat method   Subcooling method                        1   Compressor suction temperature   Liquid line temperature at the               inlet to expansion device       2   Compressor suction pressure   Condenser outlet pressure       3   Outdoor condenser coil entering air           temperature       4   Indoor returning wet bulb           temperature                  
 
         [0004]     To facilitate the superheat method, the manufacturer provides a table containing the superheat values corresponding to different combinations of indoor return air wet bulb temperatures and outdoor dry bulb temperatures for a properly charged system. This charging procedure is an empirical technique by which the installer determines the charge level by trial-and-error. The field technician has to look up in a table to see if the measured superheat falls in the correct ranges specified in the table. Often the procedure has to be repeated several times to ensure the superheat stays in a correct range specified in the table. Consequently this is a tedious test procedure, and difficult to apply to air conditioners of different makers, or even for equipment of the same maker where different duct and piping configurations are used. In addition, the calculation of superheat or subcool requires the measurement of compressor suction pressure, which requires intrusive penetration of pipes.  
         [0005]     In the subcooling method, as with the superheat method, the manufacturer provides a table listing the liquid line temperature required as a function of the amount of subcooling and the liquid line pressure. Once again, the field technician has to look up in the table provided to see if the measured liquid line temperature falls within the correct ranges specified in the table. Thus, this charging procedure is also an empirical, time-consuming, and a trial-and-error process.  
       SUMMARY OF THE INVENTION  
       [0006]     Briefly, in accordance with one aspect of the invention, a simple and inexpensive refrigerant charge inventory indication method and apparatus using temperature measurements only is provided for an air conditioning system.  
         [0007]     In accordance with another aspect of the invention, a hand held device includes a single temperature sensor which is used to sequentially sense the indoor wet bulb temperature, the condensing liquid line temperature and the outdoor temperature, and these temperatures are used to calculate a condenser approach temperature difference which, in turn, is compared with predetermined values to determine the refrigerant charge condition of an air conditioning system.  
         [0008]     By yet another aspect of the invention, the device includes an absorbent pad that may be moistened for purposes of sensing the indoor wet bulb temperature.  
         [0009]     By yet another aspect of the invention, the device includes a strap for securing the temperature sensor against the liquid line for sensing the condensing liquid line temperature.  
         [0010]     By yet another aspect of the invention, the device includes a microprocessor for storing the sensed temperatures, comparing them with predetermined stored values, and indicating the charge condition of the system.  
         [0011]     In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic illustration of an air conditioning system with present invention incorporated therein.  
         [0013]      FIGS. 2A-2D  are perspective views of a charge indicator device in various stages of use in accordance with one embodiment of the present invention.  
         [0014]      FIG. 3  is a flow chart indicating the method of testing for charge adequacy in accordance with the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     Referring now to  FIG. 1 , the invention is shown generally at  10  as incorporated into an air conditioning system having a compressor  11 , a condenser  12 , an expansion device  13  and an evaporator  14 . In this regard, it should be recognized that the present invention is equally applicable for use with heat pump systems.  
         [0016]     In operation, the refrigerant flowing through the evaporator  14  absorbs the heat in the indoor air being passed over the evaporator coil by the evaporator fan  16 , with the cooled air than being circulated back into the indoor air to be cooled. After evaporation, the refrigerant vapor is pressurized in the compressor  11  and the resulting high pressure vapor is condensed into liquid refrigerant at the condenser  12 , which rejects the heat in the refrigerant to the outdoor air being circulated over the condenser coil  12  by way of the condenser fan  17 . The condensed refrigerant is then expanded by way of an expansion device  13 , after which the saturated refrigerant liquid enters the evaporator  14  to continue the cooling process.  
         [0017]     In a heat pump, during cooling mode, the process is identical to that as described hereinabove. In the heating mode, the cycle is reversed with the condenser and evaporator of the cooling mode acting as an evaporator and condenser, respectively.  
         [0018]     It should be mentioned that the expansion device  13  may be a valve such as a TXV or an EXV which regulates the amount of liquid refrigerant entering the evaporator  14  in response to the superheat condition of the refrigerant entering the compressor  11 . It may also be a fixed orifice, such as a capillary tube or the like.  
         [0019]     In accordance with the present invention, there are three measured variables needed for assessing the charge level in an air conditioning system. These measured variables are liquid line temperature T liquid  outdoor temperature T OD  and indoor wet bulb temperature T wb .  
         [0020]     Each of these three temperatures are sensed with a single device having a single sensor and a microprocessor for storing these sensed temperatures, for storing predetermined algorithms and defining parameters for particular systems, and for indicating the charge status as a function of comparison of the sensed data with stored data.  
         [0021]     Referring now to  FIGS. 2A-2D , the charging device is shown generally at  21  having a generally rectangular housing with a front face  23 . Contained within the housing  22  is a microprocessor and, a ROM or other storage device for storing both sensed temperatures and predetermined characteristic data relative to various air conditioning models, as well as various algorithms that are used in comparing the predetermined data with the sensed data. Also included is circuitry for appropriately displaying the results of the charge adequacy test. These will be more fully discussed hereinafter.  
         [0022]     Extending from the upper end of the device  22  is a flange  24  which acts as a shelf for supporting both the temperature sensing device and the liquid refrigerant line from the condenser for purposes of sensing that temperature.  
         [0023]     Disposed at an inner edge on the upper side of the flange  24  is a sensor probe  26 , which is an elongate cylindrical structure with its upper portion being exposed as shown in  FIG. 2C . The sensor element that is associated with the sensor probe  26  is a thermocouple or the like, and the probe  26  is electronically connected to circuitry in the device  22  such that representative analog signals are sent to the processing circuitry within the housing  22  for processing as will be described hereinafter. It is this sensor probe that is used in sensing each of the three required temperatures, liquid line temperature T liquid , outdoor temperature T OD  and indoor wet bulb temperature T wb . The sensing of the outdoor temperature T OD  can be accomplished by simple taking the device  21  to an outdoor location and measuring the outdoor temperature with the sensor probe  26  in the condition as shown in  FIG. 2C .  
         [0024]     For purposes of sensing the indoor wet bulb temperature T wb , it is necessary to maintain the sensor probe  26  in a wet condition. This is accomplished by placing a cylindrically shaped sock  27  over the sensor probe  26  as shown in  FIG. 2B . The sock  27  is formed of an absorbent material which, when wetted, will allow for the sensing of the indoor wet bulb temperature T wb . Preferably, before the indoor wet bulb temperature T wb  is taken, the assembly as shown in  FIG. 2B , with the wetted sock, is made to undergo some movement, such as by a simple slinging motion to promote evaporation of the water from the wet sock to thereby present a proper condition for sensing the indoor wet bulb temperature T wb . Again, that sensed temperature is converted to an analog signal and sent to the circuitry within the housing  22  for processing.  
         [0025]     Finally, for purposes of measuring the third required temperature, the liquid line temperature T liquid , it is necessary to place the sensor probe  26  in direct contact with the condenser liquid line  28  as shown in  FIG. 2D . In order to maintain the direct contact relationship, a strap  29  is provided to be placed over the liquid line  28  and then tightly secured in place by a clasp  31  so as to maintain that firm position. Again, the T liquid  temperature that is sensed is indicated by an analog signal from the sensor probe  26  which is sent to the processing circuitry within the housing  22 .  
         [0026]     Referring now to the front panel  23  of the housing  22  as shown in  FIG. 2A , there are three LEDs,  32 ,  33  and  34  which provide indications to the operator as to the status of the process by which the temperatures are sensed and the signals are appropriately processed. Also provided is an activator button  36  and a reset button  37 .  
         [0027]     In operation, as shown in  FIG. 3 , the device is placed in the condition as shown in  FIG. 2B  with the wetted sock applied, and the indoor wet bulb temperature T wb  is sensed by pressing the activator button  36 . As the temperature is sensed as shown in block  41  of  FIG. 3 , an analog signal representative of the sensed temperature is passed to an A/D converter  42  which then passes a representative digital signal to the CPU  43  and to the read-only-memory  45  to be stored. At that point, the LED  32  will be lighted to indicate that this temperature has appropriately been sensed and stored.  
         [0028]     The wet sock  27  is then removed and the device as shown in  FIG. 2C  is taken to an outdoor location to sense the outdoor temperature T OD  as shown at block  44  of  FIG. 3 . Again, the analog signal representative of the outdoor temperature is sent to an A/D converter  46  which in turn sends a representative digital signal to the CPU  43  and to the read-only-memory  43  for storage. The LED  33  then lights up to indicate that this temperature has been sensed and stored as desired.  
         [0029]     Finally, the device  21  is taken to the condenser liquid line  28  and is attached to that line as shown in  FIG. 2D  such that the liquid line temperature can be sensed as shown in block  47  of  FIG. 3 . Again, a representative analog signal is sent to an A/D converter  48  which then converts the signal to representative digital signal which is passed to the CPU  43  and the read-only-memory  45  and stored. The LED  34  is then automatically lighted to indicate that this temperature has been appropriately sensed and stored.  
         [0030]     The processing of the three stored temperatures is accomplished by the CPU  43  by comparing the sensed liquid line temperature T liquid  for a given sensed outdoor temperature T OD  and indoor wet bulb temperature T wb  with an optimal liquid line temperature T optimal  for the same outdoor temperature and indoor wet bulb temperatures. These optimal values are stored in the read only memory  45  for each of various air conditioning system models as described in U.S. patent application No. (docket no.: 210 — 706) filed concurrently herewith, assigned to the assignee of the present invention and incorporated herein by reference. When the comparison has been made, the difference between the values calculated on the basis of the sensed temperatures and the values that are representative of an optimal condition will indicate whether the system is undercharged, overcharged or properly charged with refrigerant. The LEDS  32 ,  33  and  34  are then again used to indicate one of these three possibilities. That is, the circuitry is provided within the device  21  such that if the analysis indicates that a proper charge has been found, then the LED  33  will be automatically lighted. If it is found that refrigerant charge is needed in order to present an optimal condition, then the LED  32  will be lighted to indicate that refrigerant must be added. If it is found that the system is overcharged, then the LED  34  will be lighted to indicate that refrigerant must be removed.  
         [0031]     While the present invention has been particularly shown and described with reference to a preferred embodiment as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the true spirit and scope of the invention as defined by the claims.