Abstract:
A method for determining discharge pressure for a compressor operatively connected to a condenser, an expansion device, and an evaporator in a serial relationship, includes receiving information indicative of a compressor torque or compressor current; and determining a discharge pressure in response to the receiving of the information.

Description:
FIELD OF INVENTION 
     This invention relates generally to refrigerant vapor compression systems for residential or light commercial heating and refrigeration applications and, more particularly, to a method and system for determining the discharge pressure by utilizing system parameters and a torque-to-discharge pressure map during operation of the vapor compression system. 
     DESCRIPTION OF RELATED ART 
     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 a reduced system energy efficiency and premature compressor failure in some cases. 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 reduced system capacity, thus reducing system 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. Thermal over-load may also cause overheating and damage the pumping elements. 
     Charge adequacy has traditionally been checked manually by trained service technicians using pressure gauges, temperature measurements, and a pressure to refrigerant temperature relationship chart for the particular refrigerant resident in the system. For refrigerant vapor compression systems which use a thermal expansion valve (TXV), or an electronic expansion valve (EXV), the expansion valve component regulates the superheat of the refrigerant leaving the evaporator at a fixed value, while the amount of subcooling of the refrigerant exiting the condenser varies depending on the total system refrigerant charge (i.e. charge level). Consequently, in such systems, the “subcooling method” is customarily used as an indicator for charge level. In this method, the amount of subcooling, defined as the saturated refrigerant temperature at the refrigerant pressure at the outlet of the condenser coil for the refrigerant in use, also called the refrigerant condensing temperature, minus the actual refrigerant temperature measured at the outlet of the condenser coil, is determined and compared to a range of acceptance levels of subcooling. For example, a subcool temperature range between 10 and 15 degree Fahrenheit is generally regarded as acceptable in a refrigerant vapor compression system operating as a residential or light commercial air conditioner. 
     In general during the charging process, the technician measures the refrigerant pressure at the condenser outlet and the refrigerant line temperature at a point downstream with respect to refrigerant flow of the condenser coil and upstream with respect to refrigerant flow of the expansion valve, generally at the outlet of the condenser. With these refrigerant pressure and temperature measurements, the technician then refers to the pressure to temperature relationship chart for the refrigerant in use to determine the saturated refrigerant temperature at the measured pressure and calculates the amount of subcooling actually present at the current operating conditions, which is outdoor temperature, indoor temperature, humidity, indoor airflow and the like. If the measured amount of subcooling lies within the range of acceptable levels, the technician considers the system properly charged. If not, the technician will adjust the refrigerant charge by either adding a quantity of refrigerant to the system or removing a quantity of refrigerant from the system, as appropriate. 
     As operating conditions may vary widely from day to day, the particular amount of subcooling measured by the field service technician at any given time may not truly reflect the amount of subcooling present during “normal” operation of the system. As a result, this charging procedure is also an empirical, time-consuming, and a trial-and-error process subject to human error. Therefore, the technician may charge the system with an amount of refrigerant that is not the optimal amount charge for “normal” operating conditions, but rather with an amount of refrigerant that is merely within an acceptable tolerance of the optimal amount of charge under the operating conditions at the time the system is charged. 
     BRIEF SUMMARY 
     According to one aspect of the invention, a method for determining discharge pressure for a compressor operatively connected to a condenser, an expansion device, and an evaporator in a serial relationship, includes receiving information indicative of a compressor torque or compressor current; and determining a discharge pressure in response to the receiving of the information. 
     According to another aspect of the invention, a discharge pressure determination system for a compressor, includes a vapor compression system including a compressor, a condenser, an expansion device and an evaporator operatively connected in a serial relationship in a refrigerant flow circuit; and a control unit configured for receiving information indicative of a compressor torque or compressor current and for determining the discharge pressure as a function of the received information. 
     According to another aspect of the invention, A method for determining system subcooling in a vapor compression system including a compressor, a condenser, an expansion device and an evaporator operatively connected in a serial relationship in a refrigerant flow circuit, includes receiving information indicative of a compressor torque or compressor current; and determining a degree of system subcooling in response to the receiving of the information. 
     Other aspects, features, and techniques of the invention will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Referring now to the drawings wherein like elements are numbered alike in the FIGURES: 
         FIG. 1  illustrates a schematic view of a refrigerant vapor compression system according to an embodiment of the invention; and 
         FIG. 2  illustrates a schematic view of an air-conditioning system having an inverter-driven variable speed compressor according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of an HVAC system include a vapor compression-type HVAC system that utilizes information obtained from a controller, in order to estimate the compressor torque and predict the discharge pressure for the compressor. Compressor torque may be obtained in more than one way. With inverter driven compressors, compressor torque may be a direct output of the inverter such as, for example, by modulating the frequency of the electrical power delivered to a motor driving the inverter driven compressor, thereby controlling the torque applied by the motor on the inverter driven compressor. In single speed compressors using an AC or permanent split capacitor (PSC) motors, the torque may be obtained indirectly from the voltage differential, current, and phase-angle differential of the motor windings and used to infer the compressor torque. In one non-limiting example, the current is mapped to a compressor torque. From the compressor torque, a discharge pressure is calculated. Also, the calculated discharge pressure may be used, in an exemplary embodiment, to calculate the degrees of subcooling based on at least the discharge pressure. 
     The use of additional known system data such as suction pressure and compressor speed (in inverter driven or variable speed compressors) can enhance the accuracy of the discharge pressure prediction. The discharge pressure calculation is one of two or more variables utilized to facilitate the charging of the system in a “self-charging” mode and to periodically monitor the refrigerant charge in the system in a “charge monitoring” mode. In the vapor compression-type HVAC system, the torque driving the compressor is also related to the compressor motor current. Therefore, the discharge temperature determination methods described herein can use either the compressor torque or the compressor motor current in an equivalent matter. 
     Referring now to the drawings,  FIG. 1  illustrates an exemplary refrigerant vapor compression system  10  having a compressor  12  integrated with a single speed non-inverter type motor  24  such as, for example, an AC motor or a permanent split capacitor (PSC) motor, and operably connected to a control unit  32  according to an embodiment of the invention. Particularly, refrigerant vapor from compressor  12  is delivered to a condenser  14  where the refrigerant vapor is liquefied at high pressure, thereby rejecting heat to the outside air (e.g., via a condenser fan). The liquid refrigerant exiting condenser  14  is delivered to an evaporator  18  through an expansion valve  16 . In embodiments, the expansion valve  16  may be a thermostatic expansion valve or an electronic expansion valve for controlling superheat of the refrigerant. The refrigerant passes through the expansion valve  16  where a pressure drop causes the high-pressure liquid refrigerant to achieve a lower pressure combination of liquid and vapor. As the indoor air passes across evaporator  18  (e.g., via an evaporator fan), the low-pressure liquid refrigerant evaporates, absorbing heat from the indoor air, thereby cooling the air and evaporating the refrigerant. The low-pressure refrigerant is again delivered to compressor  12  where it is compressed to a high-pressure, high temperature gas, and delivered to condenser  14  to start the refrigeration cycle again. It is to be appreciated that while a specific refrigeration system is shown in  FIG. 1 , the present teachings are applicable to any refrigeration system, including a heat pump, HVAC, and chiller systems. 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. 
     Also shown in  FIG. 1 , system  10  includes a compressor  12 , which receives alternating current (AC) electrical power (for example, electrical power is a single-phase AC line power at 230V/60 Hz) from a power supply  20  on line  22 . In an embodiment, the compressor  12  is integrated with the single-speed motor  24  that provides the mechanical power necessary to drive a crankshaft (not shown) in the compressor  12  although, in another embodiment, the single-speed motor  24  may be a stand-alone induction motor for driving the crankshaft of the compressor  12 . Also, system  10  includes a control unit  32  operably connected to the compressor  12  and having a preprogrammed microprocessor for executing instructions stored in a computer readable medium. The control unit  32  executes algorithms for predicting the discharge pressure for the compressor  12  from information received about current and voltage differential. In an embodiment, the control unit  32  stores data related to current and voltage differential in the motor or compressor  12 , which is utilized to map to a compressor torque, which provides a differential pressure P Differential  across the compressor  12 . In an embodiment, the current, phase-angle differential and voltage differential for the start (or secondary) and run (or primary) windings of the compressor motor (not shown) are stored in a memory device in control unit  32  and used to infer a compressor torque. In another embodiment, other types of motors may be utilized in system  10  and currents obtained may be used to infer compressor torque for the compressor  12 . The memory device may be a ROM, an EPROM or other suitable data storage device. Specifically, the current, phase-angle and voltage differentials between the start and run windings are mapped to a compressor torque, and subsequently to a pressure differential to estimate the discharge pressure P Discharge . 
     In an exemplary embodiment, the control unit  32  receives information regarding the suction pressure P Suction  via a signal received by pressure sensor  26 , which corresponds to a refrigerant pressure entering the suction port of the compressor  12 , which is used to enhance the estimation of discharge pressure P Discharge  and to determine the system subcooling using refrigerant liquid line temperature shown below. In another exemplary embodiment, the compressor torque may be obtained from a torque transducer  34 , which is subsequently mapped to the discharge pressure of compressor  12  via an algorithm in control unit  32 . In an embodiment, the control unit  32  executes algorithms for calculating the discharge pressure P Discharge  of compressor  12  by mapping compressor torque to discharge pressure utilizing the suction pressure for the refrigerant being used. It is to be appreciated that the discharge pressure may be estimated from the compressor torque without utilizing a pressure sensor to directly provide a refrigerant pressure at the high side of the compressor  12 , thereby providing for a more cost-efficient HVAC system  10 . 
     Also shown in  FIG. 1 , system  100  includes a temperature sensor  30  that is connected with the refrigerant circuit to measure the refrigerant liquid line temperature, T Liquid , downstream with respect to refrigerant flow of the outlet of the condenser coil  14  and upstream with respect to refrigerant flow of the expansion valve  16 . In one example, the temperature sensor  30  may be a conventional temperature sensor, such as for example a thermocouple, thermistor, or similar device that is mounted on the refrigerant line through which the refrigerant is circulating. It is to be appreciated that the temperature sensor  30  operates to provide the refrigerant liquid line temperature T Liquid  and may also have dual usage as the defrost temperature for controlling the defrosting of the evaporator coil  14 , thereby eliminating an additional sensor needed for defrosting function for the evaporator coil  14 . In an embodiment, the control unit  32  calculates the discharge pressure P Discharge  using equation (1) and stores this value in the memory device on control unit  32 .
 
 P   Discharge   =a*P   Suction   +b *compressor speed+ c *(compressor torque)+ d *(compressor torque) 2   +e *(compressor torque) 3   +f *(compressor torque) 4    (1)
 
Where a, b, c, d, e, and f are empirical coefficients.
 
     Additionally, the control unit  32  stores, in a memory device, received signals from sensors  26 ,  30  as well as data related to compressor torque in estimating compressor discharge pressure P Discharge  to calculate the system subcooling. In calculating the system subcooling, the control unit  32  converts the analog signal received from the pressure sensor  26  into a digital signal and stores the resulting digital signal indicative of the respective measured or calculated refrigerant discharge pressure P Discharge . Similarly, the control unit  32  converts the analog signal received from the temperature sensor  30  into a digital signal and stores that digital signal indicative of the measured refrigerant liquid line temperature T Liquid . In operation, the control unit  32  is programmed to calculate the saturated discharge temperature T Dsat  from the discharge pressure P Discharge  by mapping values of P Discharge  to T Dsat . Additionally, the control unit  32  stores, in a memory device, received signals from sensors  26 ,  30  as well as data related to compressor torque in estimating compressor discharge pressure P Discharge  to calculate the system subcooling. In calculating the system subcooling, the control unit  32  converts the analog signal received from the pressure sensor  26  into a digital signal and stores the resulting digital signal indicative of the respective measured or calculated refrigerant discharge pressure P Discharge . Similarly, the control unit  32  converts the analog signal received from the temperature sensor  30  into a digital signal and stores that digital signal indicative of the measured refrigerant liquid line temperature T Liquid . The control unit  32  uses the saturated discharge temperature T Dsat  and the liquid line temperature T Liquid  to calculate the actual degrees of system subcooling. Also, the control unit  32  processes the signals received from sensor  30  indicative of the refrigerant liquid line temperature T Liquid , and utilizes the T Dsat  to P Discharge  map to store T Dsat  and T Liquid  in the memory device on control unit  32 . The control unit  32  is preprogrammed with the pressure to temperature relationship charts characteristic of at least the refrigerant in use in the system  10 . Knowing the saturated discharge temperature T Dsat , the control unit  32  calculates the actual degrees of system subcooling SSC using the following equation (2) and stores the actual degrees of subcooling in the memory unit.
 
 SSC=T   Dsat   −T   Liquid    (2)
 
       FIG. 2  illustrates a refrigerant vapor compression system  50  having a variable speed compressor  52  driven by a variable speed motor  68  according to an embodiment of the invention. The system  50  is substantially similar to the embodiment shown and described in  FIG. 1 , and includes refrigerant vapor from compressor  52  that is delivered to a condenser  54  where the refrigerant vapor is liquefied at high pressure, thereby rejecting heat to the outside air. The liquid refrigerant exiting condenser  54  is delivered to an evaporator  58  through an expansion valve  56 . In embodiments, the expansion valve  56  may be a thermostatic expansion valve or an electronic expansion valve for controlling super heat of the refrigerant. The refrigerant passes through the expansion valve  56  where a pressure drop causes the high-pressure liquid refrigerant to achieve a lower pressure combination of liquid and vapor. As the indoor air passes across evaporator  58 , the low-pressure liquid refrigerant absorbs heat from the indoor air, thereby cooling the air and evaporating the refrigerant. The low-pressure refrigerant is again delivered to compressor  52  where it is compressed to a high-pressure, high temperature gas, and delivered to condenser  54  to start the refrigeration cycle again. It is to be appreciated that while a specific refrigeration system is shown, the present teachings are applicable to any heating or cooling system, including a heat pump, HVAC, and chiller systems. In a heat pump, during cooling mode, the process is identical to that as described hereinabove, while in the heating mode, the cycle is reversed with the condenser and evaporator of the cooling mode acting as an evaporator and condenser, respectively. 
     As shown, system  50  includes a compressor  52  driven by an inverter drive  62 . In embodiments, the inverter drive  62  may be a variable frequency drive (VFD) or a brushless DC motor (BLDC) drive. Particularly, inverter drive  62  is operably coupled to compressor  52 , and receives an alternating current (AC) electrical power (for example, electrical power is a single-phase AC line power at 230V/60 Hz) from a power supply  60  and outputs electrical power on line  66  to a variable speed motor  68 . The variable speed motor  68  provides mechanical power to drive a crankshaft of the compressor  62 . In an embodiment, the variable speed motor  68  may be integrated inside the exterior shell of the compressor  62 . Inverter drive  62  includes solid-state electronics to modulate the frequency of electrical power on line  66 . In an embodiment, inverter drive  62  converts the AC electrical power, received from supply  60 , from AC to direct current (DC) using a rectifier, and then converts the electrical power from DC back to a pulse width modulated (PWM) signal, using an inverter, at a desired PWM frequency in order to drive the motor  68  at a motor speed associated with the PWM DC frequency. For example, inverter drive  62  may directly rectify electrical power with a full-wave rectifier bridge, and may then chop the electrical power using insulated gate bipolar transistors (IGBT&#39;s) or thyristors to achieve the desired PWM frequency. In embodiments, other suitable electronic components may be used to modulate the frequency of electrical power from power supply  60 . Further, control unit  64  includes a processor for executing an algorithm used control the PWM frequency that is delivered on line  66  to the motor  68 . By modulating the PWM frequency of the electrical power delivered on line  66  to the electric motor  68 , control unit  64  thereby controls the torque applied by motor  68  on compressor  52  there by controlling its speed, and consequently the capacity, of compressor  52 . 
     Also shown, the control unit  64  includes a computer readable medium for storing data in a memory unit related to estimating compressor discharge pressure (P Discharge ) from compressor and refrigeration system parameters. In embodiments, the control unit  64  stores information related to compressor torque as well as line voltages, compressor motor current, and compressor speed obtained from inverter drive  62 . It is to be appreciated that the compressor torque is also related to the compressor motor current and, in embodiments, the discharge temperature determination methods described herein can use either the compressor torque or the compressor motor current in an equivalent matter. 
     In an exemplary embodiment, the discharge pressure P Discharge  may be obtained from the motor torque of a variable speed compressor that is mapped to P Discharge . In another embodiment, the control unit  64  receives information regarding the suction pressure P Suction  via a signal received by pressure sensor  70 , which corresponds to the refrigerant pressure entering the suction port of the compressor  52 . P Suction  is used to enhance the estimation of discharge pressure P Discharge . Control unit  64  includes a processor for executing instructions necessary for performing algorithms for mapping compressor discharge pressure P Discharge  from suction pressure P Suction , compressor torque, and compressor speed. In another embodiment, the compressor torque may be obtained from a torque transducer  76  that is subsequently used to map to the discharge pressure P Discharge  of compressor  52  via an algorithm in control unit  64 . In an embodiment, the control unit  64  calculates the discharge pressure P Discharge  using equation (3) and stores this value in the memory unit:
 
 P   Discharge   =a*P   Suction   +b *compressor speed+ c *(compressor torque)+ d *(compressor torque)+ e *(compressor torque) 3   +f *(compressor torque) 4    (3)
 
Where a, b, c, d, e, and f are empirical coefficients.
 
     In an embodiment, sensor  74  is operably connected with the refrigerant circuit to measure the refrigerant liquid temperature, T Liquid , downstream with respect to refrigerant flow of the outlet of the condenser coil  54  and upstream with respect to refrigerant flow of the expansion valve  56 . It is to be appreciated that the temperature sensor  74  may be a conventional temperature sensor, such as for example a thermocouple, thermistor, or similar device that is mounted on the refrigerant line through which the refrigerant is circulating. It is to be appreciated that the temperature sensor  74  also operates to provide the defrost temperature for controlling the defrosting of the evaporator coil  58 . 
     Additionally, the control unit  64  stores, in a memory device, received signals from sensors  70 ,  74  as well as data related to compressor torque in estimating compressor discharge pressure P Discharge  to calculate the system subcooling. In calculating the system subcooling, the control unit  64  converts the analog signal received from the pressure sensor  70  into a digital signal and stores the resulting digital signal indicative of the respective measured or calculated refrigerant discharge pressure P Discharge . Similarly, the control unit  64  converts the analog signal received from the temperature sensor  74  into a digital signal and stores that digital signal indicative of the measured refrigerant liquid temperature T Liquid . In operation, the control unit  64  is programmed to calculate the saturated discharge temperature T Dsat  from the discharge pressure P Discharge  by mapping values of P Discharge  to T Dsat . Additionally, the control unit  64  stores, in a memory device, received signals from sensors  70 ,  74  as well as data related to compressor torque in estimating compressor discharge pressure P Discharge  to calculate the system subcooling. In calculating the system subcooling, the control unit  64  converts the analog signal received from the pressure sensor  70  into a digital signal and stores the resulting digital signal indicative of the respective measured or calculated refrigerant discharge pressure P Discharge . Similarly, the control unit  64  converts the analog signal received from the temperature sensor  74  into a digital signal and stores that digital signal indicative of the measured refrigerant liquid temperature T Liquid . The control unit  64  uses the saturated discharge temperature T Dsat  and the liquid line temperature T Liquid  to calculate the actual degrees of system subcooling. Also, the control unit  64  processes the signals received from sensor  74  indicative of the refrigerant liquid temperature T Liquid , and the calculated saturated discharge temperature T Dsat  and stores the processed data in the memory device on control unit  64 . The memory device may be a ROM, an EPROM or other suitable data storage device. The control unit  64  is preprogrammed with the pressure to temperature relationship charts characteristic of at least the refrigerant in use in the system  50 . Knowing the saturated discharge temperature T Dsat , the control unit  64  calculates the actual degrees of system subcooling SSC using the following equation (4) and stores the actual degrees of subcooling in the memory unit.
 
 SSC=T   Dsat   −T   Liquid    (4)
 
     The technical effects and benefits of embodiments relate to an HVAC having an inverter driven variable speed compressor that utilizes information from the inverter related to the compressor torque, compressor speed, and suction pressure in order to estimate the discharge pressure of a compressor without utilizing a pressure sensor for measuring the high side discharge pressure of the compressor. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, alterations, substitutions, or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while various embodiment of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.