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.
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.
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.
Referring now to the drawings wherein like elements are numbered alike in the FIGURES:
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,
Also shown in
In an exemplary embodiment, the control unit 32 receives information regarding the suction pressure PSuction 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 PDischarge 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 PDischarge 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
PDischarge=a*PSuction+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 PDischarge 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 PDischarge. 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 TLiquid. In operation, the control unit 32 is programmed to calculate the saturated discharge temperature TDsat from the discharge pressure PDischarge by mapping values of PDischarge to TDsat. 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 PDischarge 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 PDischarge. 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 TLiquid. The control unit 32 uses the saturated discharge temperature TDsat and the liquid line temperature TLiquid 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 TLiquid, and utilizes the TDsat to PDischarge map to store TDsat and TLiquid 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 TDsat, 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=TDsat−TLiquid (2)
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'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 (PDischarge) 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 PDischarge may be obtained from the motor torque of a variable speed compressor that is mapped to PDischarge. In another embodiment, the control unit 64 receives information regarding the suction pressure PSuction via a signal received by pressure sensor 70, which corresponds to the refrigerant pressure entering the suction port of the compressor 52. PSuction is used to enhance the estimation of discharge pressure PDischarge. Control unit 64 includes a processor for executing instructions necessary for performing algorithms for mapping compressor discharge pressure PDischarge from suction pressure PSuction, 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 PDischarge of compressor 52 via an algorithm in control unit 64. In an embodiment, the control unit 64 calculates the discharge pressure PDischarge using equation (3) and stores this value in the memory unit:
PDischarge=a*PSuction+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, TLiquid, 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 PDischarge 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 PDischarge. 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 TLiquid. In operation, the control unit 64 is programmed to calculate the saturated discharge temperature TDsat from the discharge pressure PDischarge by mapping values of PDischarge to TDsat. 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 PDischarge 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 PDischarge. 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 TLiquid. The control unit 64 uses the saturated discharge temperature TDsat and the liquid line temperature TLiquid 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 TLiquid, and the calculated saturated discharge temperature TDsat 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 TDsat, 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=TDsat−TLiquid (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.
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WO2013/101701 | 7/4/2013 | WO | A |
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