Abstract:
A refrigerant service system according to the disclosure includes an inlet port configured to connect to an air conditioning system, a recovery valve fluidly connected to the inlet port, an accumulator fluidly connected to the recovery solenoid valve and including a pressure transducer configured to generate an electronic signal corresponding to a pressure in the accumulator, and a controller. The controller is configured to determine a target pressure for the accumulator based upon a condition of the refrigerant, obtain a current pressure in the accumulator from the pressure transducer, and to operate the recovery valve based upon the accumulator target pressure to control flow of refrigerant from the air conditioning system to the accumulator based upon the obtained current pressure and the determined target pressure for the accumulator.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of priority to co-pending U.S. provisional application No. 61/911,654, filed on Dec. 4, 2013, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates generally to refrigeration systems, and more particularly to refrigerant recovery systems for refrigeration systems. 
       BACKGROUND 
       [0003]    Air conditioning systems are currently commonplace in homes, office buildings and a variety of vehicles including, for example, automobiles. Over time, the refrigerant included in these systems becomes depleted and/or contaminated. As such, in order to maintain the overall efficiency and efficacy of an air conditioning system, the refrigerant included therein is periodically replaced or recharged. 
         [0004]    Portable carts, also known as recover, recycle, recharge (“RRR”) refrigerant service carts or air conditioning service (“ACS”) units, are used in connection with servicing refrigeration circuits, such as the air conditioning unit of a vehicle. The portable machines include hoses coupled to the refrigeration circuit to be serviced. A vacuum pump and compressor operate to recover refrigerant from the vehicle&#39;s air conditioning unit, flush the refrigerant, and subsequently recharge the system from a supply of either recovered refrigerant and/or new refrigerant from a refrigerant tank. 
         [0005]    Refrigerant vapor entering the ACS unit is first passed through a filter and dryer unit to remove contaminants and moisture from the recovered charge and then through an accumulator to remove oil entrained in the refrigerant from the air conditioning system. The refrigerant is then pressurized by a compressor before it is stored in a storage tank. 
         [0006]    In typical ACS units, the pressure of the refrigerant flowing into the accumulator is regulated by an expansion valve upstream of the accumulator. The expansion reduces the pressure of the incoming refrigerant, which serves to change the state of the refrigerant from a liquid to a gas. Since the oil boils at a lower pressure than the refrigerant at a given temperature, the oil remains in a liquid state and is separated from the vaporized refrigerant. The refrigerant exiting the accumulator must be in the vapor state to prevent liquid refrigerant from entering the compressor, which can cause damage to the compressor. The expansion valve typically sets the pressure in the accumulator as a constant value, which is near the saturated vapor pressure of the refrigerant at the coldest ambient temperature at which the unit is allowed to be operated. For example, in a typical ACS unit, the accumulator is pressurized to 35 psi, which is slightly below the saturated vapor pressure of R134a at 50° F. 
         [0007]    Operating the accumulator in an ACS unit at a higher pressure reduces recovery time, increases recovery efficiency, and improves oil separation performance. What is needed, therefore, is an ACS unit which operates at varying operating accumulator pressures in order to optimize recovery performance. 
       SUMMARY 
       [0008]    In a first embodiment, a refrigerant service system according to the disclosure comprises an inlet port configured to connect to an air conditioning system, a recovery valve fluidly connected to the inlet port, an accumulator fluidly connected to the recovery solenoid valve and including a pressure transducer configured to generate an electronic signal corresponding to a pressure in the accumulator, and a controller. The controller is operable to determine a target pressure for the accumulator based upon at least one sensed condition of the refrigerant, obtain a current pressure in the accumulator from the pressure transducer, and to operate the recovery valve based upon the accumulator target pressure to control flow of refrigerant from the air conditioning system to the accumulator based upon the obtained current pressure and the determined target pressure for the accumulator. Since the controller operates the valve based upon the determined target pressure in the accumulator, the accumulator is operated at a greater pressure than prior art systems. As a result, the refrigerant in the air conditioning system can be recovered in less time that previous recovery systems. 
         [0009]    In another embodiment, the controller is further configured to operate the recovery valve to open in response to the obtained current pressure in the accumulator being less than the determined accumulator target pressure, and to operate the recovery valve to close in response to the obtained current pressure being greater than the determined accumulator target pressure. The controller therefore advantageously retains the pressure in the accumulator close to the target pressure by operation of the valve. 
         [0010]    In yet another embodiment, the refrigerant service system further comprises temperature sensor located in the accumulator and configured to generate a temperature signal corresponding to a temperature of the refrigerant in the accumulator. The controller is further configured to obtain the temperature signal from the temperature sensor and to determine the target pressure in the accumulator based upon the temperature of the refrigerant in the accumulator. Determining the target pressure based on an actual temperature in the accumulator enables an accurate determination of the vapor pressure in the accumulator and an accurate setting of the target pressure. 
         [0011]    In a further embodiment, the refrigerant service system includes an ambient temperature sensor configured to generate an ambient temperature signal corresponding to an ambient temperature. The controller is configured to obtain the ambient temperature signal and determine the target pressure in the accumulator based upon the ambient temperature signal. 
         [0012]    In some embodiments, the controller is further configured to obtain at least two pressure readings from the pressure transducer in the accumulator, determine a rate of change of the accumulator pressure based upon the at least two pressure readings, and determine the target pressure in the accumulator based upon the determined rate of change of the accumulator pressure. The controller advantageously determines the target pressure without requiring any additional sensors. 
         [0013]    In another embodiment, the refrigerant recovery system includes a refrigerant storage vessel fluidly connected downstream of the accumulator such that the recovered refrigerant can be stored in the refrigerant storage vessel. 
         [0014]    In a further embodiment, a scale is configured to generate a mass signal corresponding to a sensed mass of the refrigerant storage vessel. The controller is further configured to obtain at least two sensed mass readings from the scale, determine a mass flow rate of refrigerant flowing into the refrigerant storage vessel as a function of the at least two sensed mass readings, and determine the target pressure in the accumulator based upon the determined mass flow rate of refrigerant flowing into the refrigerant storage vessel. The controller is able to determine quickly and accurately whether the accumulator pressure exceeds the target pressure based upon the rate of change of the refrigerant storage vessel mass. Furthermore, some prior art refrigerant service systems include a scale configured to measure the weight of the refrigerant storage vessel for other purposes, such that no additional equipment would be needed for the controller to determine the target pressure in this embodiment. 
         [0015]    In another embodiment, the refrigerant service system includes a temperature sensor located at the refrigerant storage vessel and configured to generate a temperature corresponding to a sensed temperature of the refrigerant in the refrigerant storage vessel. The controller is configured to obtain at least two temperature readings from the temperature sensor, determine a rate of temperature change of the refrigerant in the refrigerant storage vessel based upon the at least two temperature readings, and determine the target pressure in the accumulator based upon the determined rate of temperature change of the refrigerant in the refrigerant storage vessel. The controller is able to determine quickly and accurately whether the accumulator pressure exceeds the target pressure based upon the rate of change of the refrigerant storage vessel temperature. 
         [0016]    In a second embodiment according to the disclosure, a method of recovering refrigerant from an air conditioning system comprises determining an accumulator target pressure for an accumulator based upon a sensed condition of refrigerant, obtaining a current pressure in the accumulator from a pressure transducer configured to sense a pressure in the accumulator, and operating a recovery valve positioned in a fluid line between the accumulator and the air conditioning system and configured to control flow of refrigerant from the air conditioning system to the accumulator based upon the obtained current pressure signal and the determined target pressure for the accumulator. Since the recovery valve based upon the determined target pressure in the accumulator, the accumulator is operated at a greater pressure than prior art systems. As a result, the refrigerant in the air conditioning system can be recovered in less time that previous recovery systems. 
         [0017]    In another embodiment according to the disclosure, the operating of the recovery valve further comprising opening the recovery valve in response to the obtained current pressure in the accumulator being less than the determined accumulator target pressure, and closing the recovery valve in response to the obtained current pressure being greater than the determined accumulator target pressure. The pressure in the accumulator is therefore advantageously retained close to the target pressure by operation of the valve. 
         [0018]    In a further embodiment, the method includes obtaining a temperature in the accumulator from a temperature sensor located in the accumulator, and determining the target pressure in the accumulator based upon the obtained temperature in the accumulator. Determining the target pressure based on an actual temperature in the accumulator enables an accurate determination of the vapor pressure in the accumulator and an accurate setting of the target pressure. 
         [0019]    In another embodiment according to the disclosure, the method further comprises obtaining at least two pressure readings from the pressure transducer in the accumulator, determining a rate of change of the accumulator pressure based upon the at least two pressure readings, and determining the target pressure in the accumulator based upon the determined rate of change of the accumulator pressure. The target pressure is advantageously determined without requiring any additional sensors. 
         [0020]    In some embodiments, the method includes obtaining at least two sensed mass readings of a refrigerant storage vessel fluidly connected downstream of the accumulator from a scale configured to sense a mass of the refrigerant storage vessel, determining a mass flow rate of refrigerant flowing into the refrigerant storage vessel as a function of the at least two sensed mass readings, and determining the target pressure in the accumulator based upon the determined mass flow rate of the refrigerant flowing into the refrigerant storage vessel. The determination whether the accumulator pressure exceeds the target pressure can be performed quickly and accurately based upon the rate of change of the refrigerant storage vessel mass. Furthermore, some prior art refrigerant service systems include a scale configured to measure the weight of the refrigerant storage vessel for other purposes, such that no additional equipment would be needed for the determination of the target pressure. 
         [0021]    In a further embodiment according to the disclosure, the method includes obtaining at least two temperature readings corresponding to a temperature of refrigerant in a refrigerant storage vessel fluidly connected downstream of the accumulator from a temperature sensor located at the refrigerant storage vessel, determining a rate of temperature change of the refrigerant in the refrigerant storage vessel based upon the at least two temperature readings, and determining the target pressure in the accumulator based upon the determined rate of temperature change of the refrigerant in the refrigerant storage vessel. The method enables quick and accurate determination of whether the accumulator pressure exceeds the target pressure based upon the rate of change of the refrigerant storage vessel temperature. 
         [0022]    In a third embodiment according to the disclosure, a refrigerant service system comprises an inlet port configured to connect to an air conditioning system, a recovery valve fluidly connected to the inlet port, an ambient temperature sensor configured to generate an ambient temperature signal corresponding to an ambient temperature of the refrigerant service system, an accumulator fluidly connected to the recovery valve and including a pressure transducer configured to generate an electronic signal corresponding to a pressure in the accumulator, and a controller. The controller is operable to determine a target pressure for the accumulator based on the ambient temperature, to obtain a current pressure in the accumulator from the pressure transducer, and to operate the recovery valve based upon the accumulator target pressure to control flow of refrigerant from the air conditioning system to the accumulator as a function of the obtained current pressure and the determined target pressure for the accumulator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is an illustration of an air conditioning service (“ACS”) machine. 
           [0024]      FIG. 2  is a schematic view of the ACS machine of  FIG. 1 . 
           [0025]      FIG. 3  is a schematic view of the control components of the ACS machine of  FIG. 1 . 
           [0026]      FIG. 4  is a process diagram of a method of operating an ACS machine during a recovery operation. 
           [0027]      FIG. 5  is a process diagram of a method of determining the target pressure at which to operate the accumulator of an ACS machine during a recovery operation. 
           [0028]      FIG. 6  is a process diagram of another method of determining the target pressure at which to operate the accumulator of an ACS machine during a recovery operation. 
           [0029]      FIG. 7  is a process diagram of another method of determining the target pressure at which to operate the accumulator of an ACS machine during a recovery operation. 
           [0030]      FIG. 8  is a process diagram of another method of determining the target pressure at which to operate the accumulator of an ACS machine during a recovery operation. 
           [0031]      FIG. 9  is a process diagram of yet another method of determining the target pressure at which to operate the accumulator of an ACS machine during a recovery operation. 
           [0032]      FIG. 10  is a graph showing the accumulator pressure versus time for a recovery processes performed at a target pressure of 35 psi and a recovery process performed at a target pressure of 95 psi. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    For the purposes of promoting an understanding of the principles of the embodiments described herein, reference is now made to the drawings and descriptions in the following written specification. No limitation to the scope of the subject matter is intended by the references. This disclosure also includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the described embodiments as would normally occur to one skilled in the art to which this document pertains. 
         [0034]      FIG. 1  is an illustration of an air conditioning service (“ACS”) unit  10 . The ACS unit  10  includes a refrigerant container or internal storage vessel (“ISV”)  12 , a controller  20 , a housing  23 , and an input/output unit  30 . The housing includes an external temperature sensor  21  configured to sense an ambient temperature outside the ACS unit  10 . Hose connections  14  (only one is shown in  FIG. 1 ) protrude from the housing  23  to connect to an A/C system and facilitate transfer of refrigerant to and from the ACS unit  10 . 
         [0035]    The ISV  12  is configured to store refrigerant for the ACS unit  10 . No limitations are placed on the kind of refrigerant that may be used in the ACS system. As such, the ISV  12  is configured to accommodate any refrigerant that is desired to be collected. In some embodiments, the ISV  12  is particularly configured to accommodate refrigerants that are commonly used in the A/C systems of vehicles (e.g., cars, trucks, boats, planes, etc.), for example R-134a, CO 2 , or R1234yf. The ISV  12  includes an ISV scale  11  configured to sense the weight of the ISV tank  12 . The ISV further includes an ISV temperature sensor  18  configured to sense a temperature of the ISV tank  12 . In some embodiments, the temperature sensor  18  is placed on the outside of the ISV  12 , while in other embodiments the sensor  18  is mounted inside the ISV  12 . In some embodiments, the ACS unit has multiple ISV tanks configured to store different refrigerants. Each independent ISV in one embodiment includes a separate scale and temperature sensor. In other embodiments, the independent ISV tanks are all weighed by a single ISV scale. 
         [0036]    Further details of the ACS system  10  are described with reference to  FIG. 2 , which is a schematic diagram of the ACS system  10  of  FIG. 1 . The ACS system  10  includes a bulkhead manifold  104 , a top manifold  108 , a lower manifold  112 , a compressor  116 , and an ISV assembly  120 . The bulkhead manifold  104  has a high-side service hose  124  with a high-side coupler  128  and a low-side service hose  132  with a low-side coupler  136 . The high-side and low-side service hoses  124 ,  132 , respectively, are configured to attach to high-side and low-side service ports of an air conditioning system, and each of the service hoses  124 ,  132  are connected to a respective hose connection  14  ( FIG. 1 ). The bulkhead manifold  104  routes the high-side service hose  124  to a high-side bulkhead hose  140  and the low-side service hose  132  to a low-side bulkhead hose  144 . The high-side and low-side bulkhead hoses  140 ,  144  each connect the bulkhead manifold  104  to the top manifold  108 . 
         [0037]    The top manifold  108  includes a high-side inlet valve  156 , which is connected to the high-side bulkhead hose  140 , and a low-side inlet valve  160 , which is connected to the low-side bulkhead hose  144 . The inlet valves  156 ,  160  both connect to a recovery valve  164 , which is connected to a manifold connection tube  168 . The manifold connection tube  168  fluidly couples the top manifold  108  to the lower manifold  112 . 
         [0038]    The lower manifold  112  includes an accumulator  172  having an accumulator pressure transducer  176  configured to sense the pressure in the accumulator  172 , an accumulator temperature sensor  180  configured to sense the temperature in the accumulator  172 , and a heat exchanger  184 . The lower manifold further includes a filter and dryer unit  188  and a compressor oil separator  192 . 
         [0039]    The ISV assembly  120  includes the ISV tank  12  having the ISV temperature sensor  18 , and the ISV scale  11 . The tank vapor hose  196  delivers the refrigerant vapor from the lower manifold  112  to the ISV assembly  120  for storage in the ISV tank  12 . 
         [0040]      FIG. 3  is a schematic diagram of the controller  20  and the components communicating with the controller  20  in the ACS system  10 . Operation and control of the various components and functions of the ACS system  10  are performed with the aid of the controller  20 . The controller  20  is implemented with a general or specialized programmable processor  208  that executes programmed instructions. In some embodiments, the controller includes more than one general or specialized programmable processor. The instructions and data required to perform the programmed functions are stored in a memory unit  204  associated with the controller  20 . The processor  208 , memory  204 , and interface circuitry configure the controller  20  to perform the functions described above and the processes described below. These components can be provided on a printed circuit card or provided as a circuit in an application specific integrated circuit (ASIC). Each of the circuits can be implemented with a separate processor or multiple circuits can be implemented on the same processor. Alternatively, the circuits can be implemented with discrete components or circuits provided in VLSI circuits. Also, the circuits described herein can be implemented with a combination of processors, ASICs, discrete components, or VLSI circuits. 
         [0041]    The external temperature sensor  21 , ISV temperature sensor  18 , and accumulator temperature sensor  180  are configured to transmit electronic signals representing the respective sensed temperatures to the controller  20 . The accumulator pressure transducer  176  transmits electronic signals representing the sensed pressure in the accumulator  172  to the controller  20  and the ISV scale  11  transmits electronic signals representing the sensed mass of the ISV  12  to the controller  20 . In various different embodiments, the ACS unit  10  does not include all of the sensors  21 ,  18 ,  180 ,  176 , and  11 . In such embodiments, the ACS unit  10  can be configured with any desired combination of an external temperature sensor  21 , an ISV temperature sensor  18 , an accumulator temperature sensor  180 , an accumulator pressure transducer  176 , and an ISV scale. 
         [0042]    The controller  20  is electrically connected to and configured to receive the temperature signals from the temperature sensors  18 ,  21 , and  180 , receive the pressure signal from the pressure transducer  176 , and receive the ISV mass signal from the ISV scale  11 . The signals from the sensors and transducers are transmitted when requested by the controller  20  or are sent continuously or on a predetermined basis, such as every 30 seconds, minute, 5 minutes, 15 minutes, 30 minutes, hour, etc. 
         [0043]    The signals received by the controller  20  are stored in the memory  204  of the controller  20 . The processor  208  transmits signals to operate the high-side inlet valve  156 , the low-side inlet valve  160 , and the recovery valve  164  based on the sensor signals and control algorithms stored in the memory  204  of the controller  20 . The controller is also connected to the input/output device  30  to enable a user to input parameters and activate operating algorithms for the controller  20 , and to enable the controller to display information to the user of the ACS unit  10 . 
         [0044]      FIG. 4  illustrates a method  300  for operating an ACS system, such as the ACS unit  10  described above with reference to  FIGS. 1-3 , during a recovery operation. The processor  208  is configured to execute programmed instructions stored in the memory  204  to operate the components in the ACS unit  10  to implement the method  300 . The method begins with the controller determining the target pressure (block  304 ). The target pressure is determined from a temperature reading in the accumulator, the ambient temperature, the rate of pressure change in the accumulator, the rate of change of mass in the ISV, and/or the rate of temperature change in the ISV. The target pressure is generally less than or equal to the saturated vapor pressure of the refrigerant at the temperature in the accumulator. Various methods of determining the target pressure are discussed in more detail below with reference to  FIGS. 5-9 . 
         [0045]    Next, the controller obtains the pressure of the accumulator (block  308 ). The pressure in the accumulator can be determined by the pressure transducer in the accumulator sensing the pressure in the accumulator and transmitting a signal representing the accumulator pressure to the controller. In some embodiments, the controller recalls a pressure value stored in the memory. The controller receives the accumulator pressure signal and compares the accumulator pressure with the target pressure (block  312 ). If the sensed accumulator pressure is greater than the target pressure, then the recovery valve is closed to reduce the pressure in the accumulator to the target pressure (block  316 ) and the process repeats from block  304 . Since the target pressure is less than or equal to the saturated vapor pressure of the refrigerant, the accumulator operates such that the refrigerant exiting the accumulator is predominantly or entirely in the vapor state. If the sensed accumulator pressure is less than the target pressure (block  320 ), then the controller operates the recovery valve to open (block  324 ), increasing the pressure in the accumulator to improve recovery efficiency. The process then continues from block  304 . If the accumulator pressure is equal to the target pressure, then the process continues from block  304  without adjusting the operation of the recovery valve. 
         [0046]    While the above method for controlling the pressure in the accumulator is described with reference to a simple control loop, the reader should appreciate that there are other ways in which the target pressure can be used to regulate the use of the recovery valve. For example, in a system having variable-position recovery valve, the relationship between the current accumulator pressure and the target pressure can be used to determine the degree of opening of the variable position valve. In some embodiments, proportional-integral-derivative (PID) control is used to more accurately retain the accumulator pressure at the target pressure. In some embodiments, a PID controller is used with a variable-position recovery valve to regulate the pressure in the accumulator. 
         [0047]    As discussed above, there are numerous methods for determining the target pressure in the accumulator.  FIGS. 5-9  each illustrate a different method of determining and/or adjusting the target pressure of the accumulator. 
         [0048]      FIG. 5  illustrates a process  330  of determining target pressure in the accumulator using the temperature in the accumulator. The processor  208  is configured to execute programmed instructions stored in the memory  204  to operate the components in the ACS unit  10  to implement the method  330 . The method  330  begins with the accumulator temperature sensor sensing the temperature in the accumulator (block  334 ). The sensor transmits a signal representing the sensed temperature to the controller. In some embodiments, the temperature in the accumulator is stored in the memory, and the controller is configured to recall the stored temperature from the memory instead of receiving the signal directly from the sensor. Once the controller receives the accumulator temperature signal the target pressure is determined for the sensed temperature (block  338 ). In one embodiment, the target pressure is the saturated vapor pressure of the refrigerant used in the system at the accumulator temperature. In another embodiment, the target pressure is less than the saturated vapor pressure by a predetermined amount to ensure that the accumulator operates below the saturated vapor pressure of the refrigerant. After the target pressure is determined, the process continues at box  308  to operate the recovery valve as discussed above. 
         [0049]      FIG. 6  illustrates another method  350  for determining the target accumulator pressure in an ACS system, which can be performed by the processor  208  executing programmed instructions stored in memory  204 , by using the ambient temperature outside the ACS unit. The method  350  begins with the external sensor sensing the ambient temperature outside the ACS system (block  354 ) and transmitting a signal representing the ambient temperature to the controller. In some embodiments, the ambient temperature is stored in the memory, and the controller is configured to recall the stored temperature from the memory instead of receiving the signal directly from the sensor. The controller receives the ambient temperature signal and estimates the accumulator temperature based on the ambient temperature (block  358 ). In some embodiments, the accumulator temperature is estimated by adding or subtracting an empirically determined constant to the sensed exterior temperature. In other embodiments, the accumulator temperature is assumed to be equal to the exterior temperature of the ACS unit. The controller then determines the target pressure at the estimated accumulator temperature (block  362 ). In one embodiment, the target pressure is the saturated vapor pressure of the refrigerant used in the system at the estimated temperature. In another embodiment, the target pressure is the saturated vapor pressure less a predetermined value to provide a factor of safety to the system to account for a difference between the accumulator temperature and the ambient temperature, sensing errors, sensor lag, and other errors in the system. After the target pressure is determined, the process continues at box  308  to operate the recovery valve as discussed above. The above method  350  of determining the target pressure for the accumulator is particularly in ACS units that do not have a temperature sensor in the accumulator. 
         [0050]      FIG. 7  illustrates another method  400  for determining the target pressure in the accumulator of an ACS system, such as the ACS unit  10  described above with reference to  FIGS. 1-3 , during a recovery operation. The processor  208  is configured to execute programmed instructions stored in the memory  204  to operate the components in the ACS unit  10  to implement the method  400 . The method  400  begins with the controller determining the current target pressure (block  404 ). In some embodiments, when the ACS unit is beginning to operate, the target pressure is set as a baseline value recalled from the memory of the processor. In other embodiments, the initial target pressure is determined using one of the other methods described herein. The target pressure can also be recalled from memory as the previous target pressure value determined using the method  400  once the system is operating. 
         [0051]    The method then continues by determining whether the recovery valve is open (block  408 ). If the recovery valve is open, the pressure transducer senses the pressure in the accumulator, and transmits a signal representing the accumulator pressure to the controller. In some embodiments, the accumulator pressure is stored in the memory, and the controller is configured to recall the stored accumulator pressure reading from the memory instead of receiving the signal directly from the sensor. The controller then uses the accumulator pressure signal and one or more previous pressure values recalled from the memory to determine the rate of the pressure increase in the accumulator due to the pressurized refrigerant passing through the recovery valve (block  412 ). 
         [0052]    Next, the rate of pressure increase is compared with an upper threshold (block  416 ). If there is liquid refrigerant in the accumulator, then the pressure rise in the accumulator will be greater than if there is only vapor refrigerant in the accumulator. As such, if the rate of the pressure increase in the accumulator when the valve is open is greater than a predetermined upper threshold, the target pressure in the accumulator is decreased (block  420 ) and the process advances to operation of the recovery valve using the adjusted target pressure at block  308 . In some embodiments, the predetermined upper threshold is the pressure increase rate at which there is known to be liquid in the accumulator, while in other embodiments the upper threshold is selected as a value that is less than the rate at which there is known to be liquid in the accumulator in order to provide a safety factor to account for possible measurement errors. 
         [0053]    If the rate of pressure increase is less than the predetermined upper threshold, then the process continues by comparing the rate of pressure increase with a lower threshold (block  424 ). The lower threshold is below the known value at which the refrigerant is entirely in the vapor state, and is based on the rate of pressure increase resulting from a desired minimum efficiency for the recovery operation. If the rate of pressure increase when the recovery valve is open is below the lower threshold, then the target pressure in the accumulator is increased to improve recovery efficiency (block  428 ) and the process advances to operation of the recovery valve using the adjusted target pressure at block  308 . If the rate of pressure increase is lower than the upper threshold but greater than the lower threshold, then the target pressure is not adjusted and the process continues at block  308 . In some embodiments, the upper and lower thresholds are equal, for example when a specific rate of pressure increase is desired during operation of the accumulator rather than a pressure increase rate within a range of values. 
         [0054]    If the recovery valve is not open (block  404 ), then the pressure transducer in the accumulator senses the pressure in the accumulator and transmits a signal representing the accumulator pressure to the controller. In some embodiments, the ambient temperature is stored in the memory, and the controller is configured to recall the stored temperature from the memory instead of receiving the signal directly from the sensor. Since the recovery valve is closed, the pressure in the accumulator decreases as the refrigerant leaves the accumulator while no additional refrigerant flows into the accumulator. The controller uses the accumulator pressure signal and one or more previous pressure values recalled from the memory to determine the rate of the pressure decrease in the accumulator due to the refrigerant exiting the accumulator (block  432 ). 
         [0055]    Next, the rate of pressure decrease is compared with a lower threshold (block  436 ). If there is liquid refrigerant in the accumulator, then the pressure decrease in the accumulator when the recovery valve is closed will be slower than if there is only vapor refrigerant in the accumulator. As such, if the rate of pressure decrease when the valve is closed is less than a predetermined lower threshold, the target pressure in the accumulator is decreased (block  440 ) and the process advances to operation of the recovery valve at block  308 . In some embodiments, the predetermined lower threshold is the pressure decrease rate at which there is known to be liquid in the accumulator, while in other embodiments the lower threshold is selected as a value that is greater than the rate at which there is known to be liquid in the accumulator in order to account for possible measurement errors. 
         [0056]    If the rate of pressure decrease is greater than the predetermined lower threshold, then the process continues by comparing the rate of pressure decrease with an upper threshold (block  444 ). The upper threshold is greater than a value at which it is known that the refrigerant is entirely in the vapor state, and is based on a rate of pressure decrease resulting from a desired minimum efficiency for the recovery operation. If the rate of pressure decrease when the recovery valve is closed is above the upper threshold, then the target pressure in the accumulator is increased to improve recovery efficiency (block  448 ) and the process continues at block  308  using the adjusted target pressure. If the rate of pressure decrease is less than the predetermined upper threshold but greater than the lower threshold, then the target pressure is not adjusted and the process continues at block  308 . In some embodiments, the lower and upper thresholds are equal, for example when a specific rate of pressure decrease is desired during operation of the accumulator rather than a pressure decrease rate within a range of values. The reader should appreciate that in various embodiments, some steps of the above method  400  are omitted or are performed in a different sequence than illustrated in  FIG. 7 . 
         [0057]      FIG. 8  illustrates yet another method  500  for determining target pressure in the accumulator of an ACS system, such as the ACS unit  10  described above with reference to  FIGS. 1-3 , during a recovery operation. The processor  208  is configured to execute programmed instructions stored in the memory  204  to operate the components in the ACS unit  10  to implement the method  500 . The method  500  begins with the controller determining the current target pressure (block  502 ). In some embodiments, when the ACS unit is beginning to operate, the target pressure is set as a baseline value recalled from the memory of the processor. In other embodiments, the initial target pressure is determined using one of the other methods described herein. The target pressure can also be recalled from memory as the previous target pressure value determined using the method  500  once a target pressure has been established. 
         [0058]    The ISV scale senses the mass in the ISV tank (block  504 ) and transmits a signal representing the sensed mass to the controller. In some embodiments, the ISV mass signals are recalled from the processor memory rather than being transmitted directly from the sensor. The controller receives the ISV mass signal and determines the rate of change of the ISV mass using the sensed ISV mass and a previously stored ISV mass reading recalled from memory (block  508 ). The rate of change of the mass in the ISV is then compared with an upper threshold (block  512 ). The manifold of the recovery system is assumed to be at steady state, and as a result the mass of refrigerant leaving the manifold to be stored in the ISV must be equal to the mass entering the accumulator. The rate of mass flowing into the accumulator is proportional to the pressure in the accumulator and, therefore, the increase in mass of the ISV is proportional to the pressure in the accumulator. If the increase in mass in the ISV is too high, then an excess of refrigerant is flowing into the accumulator, increasing the pressure in the accumulator, which can result in some refrigerant in the accumulator being in the liquid phase. Consequently, the rate of increase in ISV mass will be greater if there is liquid refrigerant present in the accumulator compared to only refrigerant in the vapor phase being present in the accumulator. The upper threshold, therefore, is selected based on a value at which it is known that liquid refrigerant is entering the accumulator at a critical rate indicative that liquid phase refrigerant is about to enter the compressor. In some embodiments, the upper threshold is at the critical rate, while in other embodiments the upper threshold is below the critical rate to account for measurement errors and ensure a factor of safety in the system. If the rate of ISV mass change is greater than the upper threshold, then the target pressure is decreased (block  516 ) and the recovery valve is operated with the adjusted target pressure at block  308 . 
         [0059]    If the rate of change of mass of the ISV is less than the upper threshold, then the controller compares the rate of change of the mass of the ISV with a lower threshold (block  520 ). The lower threshold is based on a rate of ISV mass increase at a minimum desired efficiency for the recovery operation. If the rate of change of the ISV mass is between the upper and lower thresholds, then the target pressure is not adjusted and the process continues at block  308 . If the rate of ISV mass increase is below the lower threshold, then the controller compares the rate of ISV mass increase to a bottom threshold (block  524 ), below which it is known that the pressure in the vehicle from which the refrigerant is being recovered has dropped below a level where only vapor phase refrigerant is being recovered. If the rate of the ISV mass increase is below the bottom threshold, the pressure of the refrigerant flowing into the accumulator is too low to cause condensation of the refrigerant in the accumulator, and the recovery valve is opened (block  528 ) for the remainder of the refrigerant recovery operation. If the rate of ISV mass increase is greater than the bottom threshold, but less than the lower threshold, then the target pressure is increased to improve recovery efficiency (block  532 ) and the method proceeds with operating the recovery valve with the adjusted target pressure at block  308 . 
         [0060]    In some embodiments, the upper and lower thresholds are equal, for example when a specific rate of mass change of the ISV is desired during operation of the accumulator rather than an ISV mass change rate within a range of values. In further embodiments, the process omits blocks  524  and  528 , and proceeds with increasing the target pressure (block  532 ) if the rate of mass change of the ISV is less than the lower threshold. The reader should appreciate that in various embodiments, certain steps of the above method  500  are omitted or are performed in a different sequence than illustrated in  FIG. 8   
         [0061]      FIG. 9  illustrates yet another method  550  for operating an ACS system, such as the ACS unit  10  described above with reference to  FIGS. 1-3 , during a recovery operation. The processor  208  is configured to execute programmed instructions stored in the memory  204  to operate the components in the ACS unit  10  to implement the method  550 . The method  550  begins with the controller determining the current target pressure (block  552 ). In some embodiments, when the ACS unit is beginning to operate, the target pressure is set as a baseline value recalled from the memory of the processor. In other embodiments, the initial target pressure is determined using one of the other methods described herein. The target pressure can also be recalled from memory as the previous target pressure value determined using the method  550  once a target pressure has been established. 
         [0062]    Next, the ISV temperature sensor senses the temperature of the ISV tank (block  554 ) and transmitting a signal representing the ISV tank temperature to the controller. In some embodiments, the ISV temperature signals are recalled from the processor memory rather than being directly transmitted from the sensor. The controller receives the ISV temperature signal and determines the rate of change of the ISV temperature using the sensed ISV temperature and a previously stored ISV temperature reading recalled from memory (block  558 ). The controller receives the ISV temperature and determines the rate of change of the ISV temperature using the sensed ISV temperature and a previously sensed temperature value stored in memory (block  558 ). The rate of change of the temperature in the ISV is then compared with an upper threshold (block  562 ). When the refrigerant is compressed in the compressor, the temperature of the refrigerant increases and the refrigerant flows through the heat exchanger to the ISV tank. If there is liquid refrigerant entering the accumulator, the heat exchanger located therein will not be able to remove the heat from the refrigerant passing to the ISV as quickly as when only vapor is entering the accumulator, and the refrigerant flowing from the heat exchanger to the ISV will therefore have a higher temperature. As a result, the rate of increase in ISV temperature will be greater if there is liquid refrigerant entering the accumulator compared to only vapor phase refrigerant entering the accumulator. The upper threshold is therefore selected based on a value at which it is known that liquid-state refrigerant is entering the accumulator. In some embodiments, the upper threshold is the temperature increase rate in the ISV at which it is known that liquid-state refrigerant is entering the accumulator, while in other embodiments, the upper threshold is below the temperature increase rate in the ISV at which it is known that liquid-state refrigerant is entering the accumulator to provide a factor of safety. If the rate of ISV temperature change is greater than the upper threshold, then the target pressure is decreased (block  566 ) and the process continues at block  308  with operating the recovery valve. In some embodiments, the upper threshold of the ISV temperature change rate is a value selected to control the temperature in the ISV, while still optimizing recovery efficiency. Excess heat in the ISV results in increased pressure in the ISV and eventually activation of a pressure relief valve (not shown) in the ISV, resulting in loss of refrigerant to the atmosphere. Thus, reducing the rate at which the ISV temperature increases by controlling the flow of refrigerant into the accumulator reduces the chance of the temperature in the ISV causing the pressure relief valve to open and waste refrigerant. 
         [0063]    If the rate of change of the ISV temperature is not greater than the upper threshold, then the controller compares the rate of change of the ISV temperature with a lower threshold (block  570 ). The lower threshold is based on a rate of change in the ISV temperature resulting from a minimum desired efficiency of the recovery operation. If the rate of change of ISV temperature change is below the lower threshold, then the controller increases the target pressure to improve recovery efficiency (block  574 ) and proceeds to operating the recovery valve with the adjusted target pressure at block  308 . If the rate of change of the ISV temperature is between the upper and lower thresholds, then the target pressure is not adjusted and the process continues at block  308 . In some embodiments, the upper and lower thresholds are equal, for example when a specific rate of temperature change of the ISV is desired during operation of the accumulator rather than an ISV temperature change rate within a range of values. The reader should appreciate that in various embodiments, some steps of the above method  550  are omitted or are performed in a different sequence than illustrated in  FIG. 9 . 
         [0064]    While each method is described above individually, the reader should appreciate that in various embodiments, the target pressure is determined using a combination of any or all of the above methods  330 ,  350 ,  400 ,  500 , and  550 . 
         [0065]      FIG. 10  illustrates a graph  600  of the accumulator pressure against time for a target pressure of 35 psi (line  604 ) and a target pressure of 95 psi (line  608 ), which corresponds to the saturation vapor pressure of R-134a at approximately 76 degrees F. As can be seen from the graph, increasing the target pressure from 35 psi to 95 psi reduces the recovery time from approximately 370 seconds down to approximately 280 seconds. 
         [0066]    It will be appreciated that variants of the above-described and other features and functions, or alternatives thereof, may be desirably combined into many other different systems, applications or methods. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art that are also intended to be encompassed by the foregoing disclosure.