Abstract:
A method of defrosting a transcritical vapor compression system having a compressor for compressing a refrigerant, a first heat exchanger for cooling the refrigerant during a cooling mode, an expansion valve for decreasing the pressure of the refrigerant, and a second heat exchanger for cooling a space during the cooling mode. The method includes attaining a superheated refrigerant condition in a defrost mode of the transcritical vapor compression system and defrosting the second heat exchanger in the defrost mode by directing the superheated refrigerant to the second heat exchanger without bypassing the first heat exchanger.

Description:
BACKGROUND 
       [0001]    The present invention relates to a method and apparatus for defrosting a heat exchanger coil of a transcritical vapor compression system. 
         [0002]    Transcritical vapor compression systems typically include a compressor, a gas cooler, an expansion valve and an evaporator. Typically, electric heaters are installed in front of a heat transfer surface of the evaporator for defrosting the evaporator heat transfer surface. When a defrost mode is initiated, a controller stops the compressor and energizes the electric heaters. The heaters are turned off, and cooling mode resumes, when the evaporator coil temperature increases. 
       SUMMARY 
       [0003]    In one aspect, the invention provides a method of defrosting a transcritical vapor compression system operable in a cooling mode and a defrost mode, the transcritical vapor compression system having a compressor for compressing a refrigerant, the compressor having a compressor inlet and a compressor outlet and operating at a first speed/frequency during the cooling mode, a first heat exchanger for cooling the refrigerant during the cooling mode, an expansion valve for decreasing the pressure of the refrigerant, the expansion valve having a variable opening, and a second heat exchanger for cooling a space during the cooling mode. The method includes directing a superheated refrigerant gas from the compressor to the first heat exchanger during the defrost mode, then directing the superheated refrigerant gas from the first heat exchanger to the expansion valve during the defrost mode, then directing the superheated refrigerant gas from the expansion valve to the second heat exchanger during the defrost mode, and defrosting the second heat exchanger with the superheated refrigerant gas. 
         [0004]    In another aspect, the invention provides a transcritical vapor compression system. The transcritical vapor compression system includes a compressor for compressing a refrigerant, the compressor having a compressor inlet and a compressor outlet. The system also includes a first heat exchanger for cooling the refrigerant, an expansion valve for decreasing the pressure of the refrigerant, the expansion valve having a variable opening, and a second heat exchanger for heating the refrigerant. The system also includes a controller programmed to decrease the speed/frequency of the compressor during a defrost mode, programmed to determine a desired superheat temperature, programmed to compare the desired superheat temperature with a measured temperature proximate the compressor inlet, and programmed to adjust the expansion valve based on the comparison between the desired superheat temperature and the measured temperature during the defrost mode. 
         [0005]    In another aspect, the invention provides a method of defrosting a transcritical vapor compression system having a compressor for compressing a refrigerant, a first heat exchanger for cooling the refrigerant during a cooling mode, an expansion valve for decreasing the pressure of the refrigerant, and a second heat exchanger for cooling a space during the cooling mode. The method includes attaining a superheated refrigerant condition in a defrost mode of the transcritical vapor compression system and defrosting the second heat exchanger in the defrost mode by directing the superheated refrigerant to the second heat exchanger without bypassing the first heat exchanger. 
         [0006]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram of a transcritical vapor compression system in accordance with the invention. 
           [0008]      FIG. 2  is a diagram of internal energy and pressure of the transcritical vapor compression system shown in  FIG. 1  during a cooling mode and during a defrost mode. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0010]      FIG. 1  illustrates a transcritical vapor compression system  10 . The transcritical vapor compression system  10  is a closed circuit single stage vapor compression cycle preferably utilizing carbon dioxide (CO 2 ) as a refrigerant, although other refrigerants suitable for a transcritical vapor compressor system may be employed. The system  10  includes a variable speed/frequency compressor  14 , a gas cooler  18 , an expansion valve  22 , an evaporator  26  and an accumulator tank  30  connected in series. A blower  36 , or blowers, move air over the gas cooler  18  for heat exchange therewith, and a blower  40 , or blowers, move air over the evaporator  26  for heat exchange therewith. Temperature sensors  42   a - 42   h  are located at the compressor inlet  1  (refrigerant temperature), the compressor outlet  2  (refrigerant temperature), the gas cooler outlet  3  (refrigerant temperature), the evaporator refrigerant inlet  4  (refrigerant temperature), the evaporator refrigerant outlet  5  (refrigerant temperature), the evaporator air inlet  6  (air temperature), the evaporator coil  7  (coil temperature), and the evaporator air outlet  8  (air temperature), respectively. Pressure sensors  46   a - 46   c  are located at the compressor inlet  1 , the compressor outlet  2 , and the gas cooler outlet  3 , respectively, for measuring refrigerant pressure. 
         [0011]    As shown schematically in  FIG. 1 , the transcritical vapor compression system  10  is controlled by a controller  50 . The controller  50  also controls the opening of the expansion valve  22 , the speed/frequency (speed or frequency) of the blowers  36 ,  40  and the speed/frequency of the compressor  14 , and receives input signals from the temperature sensors  42   a - 42   h  and the pressure sensors  46   a - 46   c , as will be described in greater detail below. 
         [0012]    In a cooling mode, refrigerant exits the evaporator coil  26  as a heated gas and is drawn into a suction port of the compressor  14 , which is preferably a variable speed/frequency compressor. The compressor  14  pressurizes and discharges heated refrigerant gas into the gas cooler  18 . In the gas cooler  18 , or heat exchanger, the heated refrigerant is cooled to a lower temperature gas as a result of a forced flow of air  34  flowing over the gas cooler  18  and generated by the blowers  36 , which are preferably variable speed blowers. The gas cooler  18  can include one or more heat exchanger coils having any suitable construction, as is known in the art. Then, the cooled refrigerant is throttled through the expansion valve  22 , such as an electronic expansion valve, and directed toward the evaporator coil  26  at a decreased pressure as a liquid-vapor mixture, or wet vapor. In the evaporator coil  26 , or heat exchanger, the cooled refrigerant is heated to a higher temperature gas as a result of a forced flow of air  38  generated by blowers  40 , such as variable speed blowers. In other words, the refrigerant passing through the evaporator coil  26  absorbs the heat from the flow of air  38  such that the flow of air  38  is cooled. The evaporator coil  26  can include one or more heat exchanger coils having any suitable construction, as is known in the art. Then, the refrigerant passes through the accumulator tank  30 , and only vapor refrigerant exits the accumulator tank  30  to the inlet of the compressor  14 . 
         [0013]    To obtain desirable refrigeration characteristics from the refrigerant, the transcritical refrigeration cycle requires higher operating pressures compared to a reverse-Rankine refrigeration cycle. With reference to  FIG. 2 , the pressure of the refrigerant in the gas cooler  18  is in the supercritical region of the refrigerant, i.e., at or above the critical temperature and critical pressure of the refrigerant. For example, the critical point of CO 2  occurs at approximately 7.38 MPa (1070 psia) and approximately 31 degrees Celsius (88 degrees Fahrenheit). In the illustrated construction, the pressure of refrigerant in the gas cooler  18  during the cooling mode is approximately 8.2 MPa (1200 psia). The pressure of refrigerant in the evaporator  26  is also higher than pressures seen in a reverse-Rankine refrigeration cycle. In the illustrated construction, the pressure of refrigerant in the evaporator  26  is approximately 2.7 MPa (390 psia). As a result, the gas cooler  18  and evaporator coil  26  employ a heavy-duty construction to withstand the higher pressures. 
         [0014]    The controller  50  is programmed to initiate a defrost mode to defrost the evaporator coils  26  periodically based on time. For example, the controller  50  is programmed to begin defrost mode every 2 hours. Other suitable time periods may be employed, such as every 4 hours, every 6 hours, or another suitable time period. 
         [0015]    The controller  50  is programmed to monitor compressor suction temperature and pressure at the compressor inlet  1  by way of temperature and pressure sensors  42   a ,  46   a , respectively, during the defrost mode. A saturated vapor curve  58  for the refrigerant is stored in the controller  50 . In the illustrated construction, with reference to  FIG. 2 , the saturated vapor curve  58  for carbon dioxide is shown. The controller  50  is programmed to calculate a saturated vapor temperature Ts based on the measured suction pressure signal from the suction pressure sensor  46   a  during the defrost mode. The controller  50  is programmed to include a predetermined offset X, such as 4 Kelvin, and to calculate a desired superheat temperature at the compressor inlet  1  for the defrost mode by adding the predetermined offset X to the calculated saturated vapor temperature Ts. The defrost mode includes a transition mode and a superheat mode. At the onset of the defrost mode, the system  10  is in the transition mode as the refrigerant transitions to superheat. When the desired superheat temperature at the compressor inlet  1  is reached, then the system  10  is in superheat mode. 
         [0016]    The controller  50  is programmed to decrease the speed or frequency of the compressor  14  down to a relatively low level, e.g., low speed/frequency, at the onset of the defrost mode, i.e., during the transition mode. Low speed/frequency is generally lower than the speed/frequency of the compressor  14  during the cooling mode. Preferably, low speed/frequency is the lowest operable speed/frequency setting for the compressor  14  greater than zero. The controller  50  is also programmed to turn the gas cooler blowers  36  off and fully open the expansion valve  22  at the onset of the defrost mode, i.e., at the onset of the transition mode. As necessary, the controller  50  is also programmed to control the speed/frequency of the gas cooler blowers  36  based on a refrigerant pressure value at the compressor outlet  2  to maintain the refrigerant pressure value below a maximum permitted pressure value. The controller  50  is programmed to control the speed/frequency of the evaporator blowers  40  (e.g., on or off, high speed, low speed, etc.) during the transition mode based on a comparison between a measured suction temperature T 1  at the compressor inlet  1  from sensor  42   a  and the desired superheat temperature (Ts+X). For example, the controller  50  is programmed such that if the measured suction temperature T 1  is not greater than (or greater than or equal to) the desired superheat temperature (Ts+X), then the expansion valve  22  is partially closed and the evaporator blowers  40  remain on. Furthermore, the controller  50  is programmed such that if the measured suction temperature T 1  is greater than (or greater than or equal to) the desired superheat temperature (Ts+X), then the evaporator blowers  40  are turned off and the expansion valve  22  is fully opened. The controller  50  is programmed such that, when the desired superheat temperature (Ts+X) at the compressor inlet  1  is reached or exceeded, the expansion valve  22  is fully opened and the evaporator blowers  40  are turned off. 
         [0017]    The controller  50  is programmed to terminate the defrost mode and initiate the cooling mode when desired conditions are reached. The controller  50  is programmed to monitor the temperature T 7  of the evaporator coil  26 , as indicated by a signal received from the temperature sensor  42   g , during the defrost mode. The controller  50  is programmed to terminate the defrost mode and initiate the cooling mode when the temperature T 7  of the evaporator coil  26 , as measured by the temperature sensor  42   g , reaches a predetermined evaporator coil temperature. In other constructions, the controller  50  may be programmed to terminate the defrost mode and initiate the cooling mode based on other desired conditions, such as duration of defrost mode, amongst others. 
         [0018]      FIG. 2  is a pressure-enthalpy diagram illustrating the saturated liquid line  54  for CO 2 , the saturated vapor line  58  for CO 2 , and the working area of the system (enthalpy vs. pressure) during the cooling mode and the defrost mode, the defrost mode being depicted as the transition mode and the superheat mode. In operation, when defrosting of the evaporator coils  26  is due, the controller  50  initiates the defrost mode. The defrost mode starts with the transition mode, in which the refrigerant increases in temperature and transitions to superheat. As shown in  FIG. 2 , the system working area moves to the right on the pressure-enthalpy diagram during the transition mode, from an area crossing the saturated liquid curve  54  and the saturated vapor curve  58  into an area on the right side of the saturated vapor curve  58 , i.e., in the superheat region. 
         [0019]    In the transition mode, the controller  50  decreases the speed or frequency of the compressor  14  down to low speed or frequency, as described above. Then, the controller turns the gas cooler blowers  36  off and maintains the speed of the evaporator blowers  40  while the opening of the expansion valve  22  is controlled to achieve the desired superheat temperature of refrigerant at the compressor inlet  1 . If the refrigerant pressure value at the compressor outlet  2  reaches or exceeds the maximum pressure value, then the controller turns on and/or increases the speed/frequency of the gas cooler blowers  36  in order to manage the pressure at the compressor outlet  2 . As described above, the desired superheat temperature is calculated by first calculating the saturated vapor temperature corresponding to the actual vapor pressure P 1  measured at the compressor inlet  1 , and then adding the predetermined offset X to the calculated saturated vapor temperature. In order to calculate the saturated vapor temperature, the controller  50  includes thermophysical property data, e.g., the saturated vapor curve, corresponding to the type of refrigerant used in the system  10 . The controller  50  looks up the saturated vapor temperature that corresponds to the measured vapor pressure P 1  for the type of refrigerant used. 
         [0020]    During the defrost mode, the hot refrigerant gas from the compressor  14  enters and exits the gas cooler  18  with a relatively small amount of cooling of the refrigerant occurring in the gas cooler  18 , i.e., substantially less cooling than in the cooling mode. The expansion valve  22  lowers the pressure of the hot gas refrigerant, and lower pressure hot gas refrigerant is produced at the outlet of the expansion valve  22 . Thus, the refrigerant in the evaporator  26  begins to transition from wet vapor to hot gas. If the refrigerant at the compressor inlet  1  has not reached the desired superheat temperature, the controller  50  partially closes the opening of the expansion valve  22  in order to achieve the desired superheat temperature. When the evaporator  26  receives a hot, or superheated, gas refrigerant, signaling the end of the transition mode and beginning of the superheat mode, the controller  50  turns the evaporator blowers  40  off and fully opens the expansion valve  22 . The controller  50  determines that the evaporator  26  is receiving a hot gas when the temperature T 1  at the compressor inlet  1  reaches or exceeds the desired superheat temperature. Heat from the hot refrigerant gas, or superheated refrigerant gas, passing through the coils of the evaporator  26  defrosts the coils of the evaporator  26 . The evaporator coil temperature increase is monitored to terminate defrost. The controller  50  terminates the defrost mode when the temperature of the evaporator coil, as indicated by the evaporator coil temperature sensor  42   g , reaches a predetermined value. When the controller  50  terminates the defrost mode, the controller  50  switches back to the cooling mode. 
         [0021]    It is to be understood that the controller  50  may include a single controller, multiple controllers or a system of controllers for controlling various aspects of the invention described herein. 
         [0022]    Thus, the invention provides, among other things, a controller programmed to defrost a transcritical vapor compression system using a superheated refrigerant and without requiring an auxiliary heater or modified piping and in which the evaporator  26  remains on the low pressure side during both the cooling and defrost modes such that the evaporator  26  need not be dimensioned to withstand transcritical pressures. Various features and advantages of the invention are set forth in the following claims.