Patent Publication Number: US-9890979-B2

Title: System and method for venting refrigerant from an air conditioning system

Description:
CLAIM OF PRIORITY 
     This application claims priority to U.S. Provisional Application Ser. No. 62/073,375 entitled “System and Method for Venting Refrigerant from an Air Conditioning System,” filed Oct. 31, 2014, the disclosure of which is hereby incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to refrigeration systems, and more particularly to refrigerant service systems for refrigeration systems. 
     BACKGROUND 
     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 in the system is periodically replaced or recharged. 
     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. In some current refrigeration systems the refrigerant, for example R134a or R1234yf, used is expensive and can be hazardous if released into the atmosphere. As such, a vacuum pump and compressor operate to recover refrigerant from the vehicle&#39;s air conditioning unit, flush the refrigerant, and subsequently store the recovered refrigerant in a refrigerant tank. The refrigerant can then be used in another refrigeration system. Recovering the refrigerant, however, requires the ACS unit to include filters, heat exchangers, a compressor, a storage tank, and a scale to weigh the storage tank. 
     Some newer air conditioning systems have begun using R744, or carbon dioxide, as an economical and eco-friendly refrigerant alternative. Removal of the R744 refrigerant from these air conditioning systems is done by venting the refrigerant to the atmosphere in a controlled manner. The R744, however, is at a very high static pressure in the air conditioning system at ambient conditions, such that the venting of the refrigerant must be controlled to prevent damage to components or elastomeric seals in the air conditioning system. What is needed, therefore, is an ACS unit that can accurately determine the flow rate of refrigerant vented from an air conditioning system during a service operation. 
     Additionally, it is advantageous to measure the total mass discharged from the air conditioning system to aid in diagnostics of the air conditioning system, for example to determine if the system has a leak. Since the R744 refrigerant is vented to atmosphere, and not captured, it is difficult or impossible in conventional ACS units to accurately determine the quantity of refrigerant removed from the air conditioning system during the venting. What is needed, therefore, is an ACS unit that can accurately determine total mass of R744 refrigerant vented from an air conditioning system during a service operation. 
     SUMMARY 
     In one embodiment, an air conditioning service system comprises an inlet port, a discharge circuit, a pressure transducer, and a controller. The inlet port is configured to connect to an air conditioning system to receive refrigerant and the pressure transducer is configured to sense a pressure at the inlet port. The discharge circuit includes a plurality of discharge lines arranged in parallel with one another. Each of the plurality of discharge lines fluidly connects the inlet port to the atmosphere through an associated orifice having a cross-sectional area to vent the refrigerant to atmosphere. The discharge unit further includes a plurality of discharge valves, each of which is associated with one of the plurality of discharge lines and is configured to open and close the associated one of the plurality of discharge lines. The controller is operably connected to the pressure transducer and to each of the plurality of discharge valves. The controller includes a memory and a processor configured to execute program instructions stored in the memory to obtain the sensed pressure at the inlet port and determine a theoretical mass flow rate through each of the plurality of discharge lines based upon the sensed pressure and the cross-sectional area of the associated orifice, and to operate selected ones of the plurality of discharge valves based upon the determined theoretical mass flow rates. 
     In some embodiments, the controller is configured to determine a first set of the plurality of discharge valves having a combined theoretical flow rate that is less than a predetermined maximum flow rate, and to operate the first set of the plurality of discharge valves to open. In a further embodiment, the controller is configured to determine the first set of the plurality of discharge valves such that the total theoretical flow rate of the valves of the first set is a maximum possible combined theoretical flow rate that is less than the predetermined maximum flow rate. 
     In yet another embodiment of the air conditioning service system, the controller is further configured to determine a first mass flow through the first set of the plurality of discharge valves during a first time period, and to store the mass flow in the memory. In some embodiments, the controller is further configured to determine a total mass by summing a plurality of mass flows determined during a venting operation. 
     A method according to the disclosure for venting refrigerant from an air conditioning system comprises sensing a pressure of a refrigerant at an inlet port of an air conditioning service system that is connected to an air conditioning system to receive refrigerant therefrom, and determining a theoretical mass flow rate through each discharge line of a plurality of discharge lines based upon the sensed pressure and a cross-sectional area of an associated orifice arranged in the discharge line, wherein the plurality of discharge lines are arranged in parallel with one another in a discharge circuit and each of the plurality of discharge lines connecting the inlet port to the atmosphere through the associated orifice. The method further comprises operating a plurality of discharge valves, each of the plurality of discharge valves being configured to open and close an associated one of the plurality of discharge lines, based upon the determined theoretical mass flow rates, and discharging refrigerant to atmosphere through selected ones of the plurality of discharge valves that are open. 
     In another embodiment the method further comprises determining a first set of the plurality of discharge valves having a combined theoretical flow rate that is less than a predetermined maximum flow rate, and the operating of the plurality of discharge valves includes operating the first set of the plurality of discharge valves to open. 
     In yet another embodiment of the method, the determining of the first set of the plurality of discharge valves includes determining the first set having a maximum possible total combined theoretical flow rate that is less than the predetermined maximum flow rate. 
     In some embodiments, the method further comprises determining a first mass flow through the first set of valves during a first time period and storing the mass flow in a memory. In one embodiment, the method further comprises determining a total mass by summing a plurality of mass flows determined during a venting operation and storing the total mass in the memory. 
     In another embodiment according to the disclosure, an air conditioning service system comprises an inlet port configured connect to an air conditioning system, and a discharge circuit including a first discharge line fluidly connecting the inlet port to the atmosphere and a first discharge valve configured to open and close the first discharge line, the first discharge line including a first orifice having a first flow area. The air conditioning service system further comprises a pressure transducer configured to sense a pressure at the inlet port and a controller operably connected to the pressure transducer and configured to obtain the sensed pressure at the inlet port. The controller is further configured to determine a first theoretical mass flow rate through the first orifice based upon the sensed pressure and the first cross-sectional area, and to operate the first discharge valve based upon the first theoretical mass flow rate. 
     In one embodiment of the air conditioning service system, the discharge circuit further comprises a second discharge line fluidly connecting the inlet port to the atmosphere and a second discharge valve configured to open and close the second discharge line, the second discharge line including a second orifice having a second flow area, and the controller is further configured to determine a second theoretical mass flow rate through the second orifice based upon the sensed pressure and the second cross-sectional area, and to operate the first and second discharge valves based upon the first and second theoretical mass flow rates. In some embodiments, the first cross-sectional area is different from the second cross-sectional area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cutaway front view of an ACS machine according to the disclosure. 
         FIG. 2  is side perspective view of the ACS machine of  FIG. 1  connected to a vehicle. 
         FIG. 3  is a schematic view of the ACS machine according to the disclosure configured to vent refrigerant to the atmosphere through control orifices. 
         FIG. 4  is a schematic view of the control components of the ACS machine of  FIG. 3 . 
         FIG. 5  is a process diagram of a method of operating an ACS machine during a venting operation. 
         FIG. 6  is a process diagram of a method of determining the total mass vented from an air conditioning system during a venting operation. 
         FIG. 7  is a graph showing the mass flow rate versus time for a simulation of a venting process. 
         FIG. 8  is a graph showing the total mass vented versus time for the simulation depicted in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     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. 
       FIG. 1  is an illustration of an air conditioning service (“ACS”) system  100 . The ACS system  100  includes a refrigerant container or internal storage vessel (“ISV”)  14 , a manifold block  16 , a compressor  18 , a control module  20 , and a housing  22 . The exterior of the control module  20  includes an input/output unit  26  for input of control commands by a user and output of information to the user. Hose connections  30  (only one is shown in  FIG. 1 ) protrude from the housing  22  to connect to service hoses that connect to an air conditioning (“A/C”) system  40  ( FIG. 2 ) and facilitate transfer of refrigerant between the ACS system  100  and the A/C system  40 . The manifold block  16  is fluidly connected to the ISV  14 , the compressor  18 , and the hose connections  30  through a series of valves, hoses, and tubes, which are discussed in detail below with reference to  FIG. 3 . 
     The ISV  14  is configured to store refrigerant for the ACS system  100 . No limitations are placed on the kind of refrigerant that may be used in the ACS system  100 . As such, the ISV  14  is configured to accommodate any refrigerant that is desired to be charged to the A/C system  40 . In some embodiments, the ISV  14  is particularly configured to accommodate one or more 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  (also known as R-744), or R-1234yf. In some embodiments, the ACS system has multiple ISV tanks configured to store different refrigerants. 
       FIG. 2  is an illustration of a portion of the air conditioning recharging system  10  illustrated in  FIG. 1  connected to the A/C system  40  of a vehicle  50 . One or more service hoses  34  connect an inlet and/or outlet port of the A/C system  40  of the vehicle  50  to the hose connections  30  (shown in  FIG. 1 ) of the ACS system  100 . 
       FIG. 3  illustrates a schematic diagram of the ACS system  100 . The ACS system  100  includes a coupling system  104 , a discharge circuit  108 , a charge circuit  112 , an injection circuit  116 , and a controller  120 . The coupling system  104  includes a high-side coupler  124  connected to a high-side pressure gauge  128 , a high-side pressure transducer  132 , and a high-side pressure relief valve  136 ; and a low-side coupler  140  connected to a low-side pressure gauge  144 , a low-side pressure transducer  148 , and a low-side pressure relief valve  152 . The low and high-side couplers  124 ,  140  include hose connections  30  ( FIG. 2 ) configured to connect to the service hoses  34 , to connect the ACS system  100  to an air conditioning system, for example air conditioning system  40  of vehicle  50 . 
     Referring back to  FIG. 3 , the discharge circuit  108  includes a vacuum pump subsystem  160  having a vacuum pump  164 , two vacuum solenoid valves  168 ,  172 , and a vacuum transducer  176 . The vacuum pump  164  is configured to produce a negative pressure in the discharge circuit  108 . 
     The discharge circuit  108  further includes a high-side inlet solenoid valve  180  and a low-side inlet solenoid valve  184 , which are connected to the high-side and low-side couplers  124 ,  140 , respectively. The outlets of the inlet valves  180 ,  184  are both connected to a joint line  186 , which splits into two discharge lines  188 ,  190 , which are arranged in parallel with one another, downstream of a connection of the joint line  186  to a line connecting to the vacuum subsystem  160 . A first system discharge solenoid valve  192  is configured to open and close the first discharge line  188 , and a first control orifice  196  are arranged in the first discharge line  188 . A second discharge solenoid valve  200  is configured to open and close the second discharge line  190 , and a second control orifice  204  is arranged in the second discharge line  190 . In one embodiment, the control orifices  196 ,  204  have different cross-sectional areas. In some embodiments, only one system discharge valve and control orifice may be included, while other embodiments may include more than two system discharge valves and corresponding control orifices and discharge lines arranged in parallel with one another. 
     The discharge lines  188 ,  190  join and connect to a system oil separator  220 . The system oil separator  220  is configured to separate the refrigerant from oil entrained in the refrigerant during normal operation of the air conditioning system. The oil flows through an oil drain solenoid valve  224  into an oil drain vessel  228 , while the refrigerant flows through a discharge passage  232 , which is open to the atmosphere. 
     The charge circuit  112  includes a high-side charge line  240  connected to the high-side coupler  124  and a low-side charge line  244  connected to the low-side coupler  140 . The charge lines  240 ,  244 , respectively, each include a check valve  248 ,  252  allowing flow only in the direction of the couplers  124 ,  140 , and a charge solenoid valve  256 ,  260  to control flow during charging. The charge lines  240 ,  244  connect to a joint charge line  264 , which includes an inflow orifice  268  configured to control the flow rate during charging, and a pressure relief valve  272  configured to prevent excess pressure from building in the charge circuit  112 . The joint charge line  264  connects to the ISV  14 , which is positioned in the ACS system on a refrigerant scale  280  configured to measure the weight of refrigerant in the ISV  14 . 
     The injection circuit  116  is connected to the high-side coupler  124  and includes an oil injection subsystem  300  and a dye injection subsystem  304 . The oil injection subsystem  300  includes a check valve  308  configured to enable flow only in the direction of the high-side coupler  124 , an oil injection solenoid valve  312  configured to regulate flow of oil, an oil vessel  316 , and an oil vessel scale  320  configured to measure the weight of the oil vessel  316 . The oil injection subsystem  300  is configured to replenish oil that is entrained in the refrigerant removed from the air conditioning system to ensure proper operation of the air conditioning system. 
     The dye injection subsystem  304  includes a check valve  324  configured to enable flow only in the direction of the high-side coupler  124 , a dye injection solenoid valve  328  configured to regulate flow of oil, a dye vessel  332 , and a dye vessel scale  336  configured to measure the weight of the dye vessel  332 . The dye injection subsystem is configured to inject dye into the air conditioning system to enable a technician to perform diagnostic operations, for example detecting leaks in the air conditioning system. 
       FIG. 4  is a schematic diagram of the controller  120  and the components operably connected to the controller  120  in the ACS system  100 . Operation and control of the various components and functions of the ACS system  100  are performed with the aid of the controller  120 . The controller  120  is implemented with a general or specialized programmable processor  352  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  356  associated with the controller  120 , which may be integral with the controller  120  (as shown in  FIG. 4 ) or may be a separate unit. The processor  352 , memory  356 , and interface circuitry configure the controller  120  to perform the functions and 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. 
     The pressure transducers  132 ,  148 ,  176  are configured to transmit electronic signals representing the sensed pressure at their respective locations to the processor  352 , and the refrigerant scale  280  and the injection scales  320 ,  336  transmit electronic signals representing the sensed weight in the ISV  14 , the oil vessel  316 , and the dye vessel  332 , respectively, to the processor  352 . The processor  352  obtains the signals from the pressure transducers  132 ,  148 ,  176  and scales  280 ,  320 ,  336  at predetermined time intervals or as necessary to perform computations, and stores relevant values from the transducers and scales in the memory  356 . 
     The processor  352  is also electrically connected to the solenoid valves  168 ,  172 ,  180 ,  184 ,  192 ,  200 ,  224 ,  256 ,  260 ,  308 ,  324 , and is configured to transmit electronic signals that instruct the valves to operate to open or close. The processor  352  is further connected to the vacuum pump  164  and is configured to transmit electronic signals to operate the vacuum pump  164  to activate and deactivate. The controller  120  also includes a timer  360 , which may be integral with the controller  120 , as illustrated in  FIG. 4 , or may be embodied as a separate timer circuit. 
     During a refrigerant servicing operation, the ACS system  100  is configured to vent the refrigerant in the air conditioning system, for example R744 (carbon dioxide), to the atmosphere. A technician connects the high-side coupler  124  and the low-side coupler  140  to the high-side and low-side ports of the air conditioning system via service hoses. The controller  120  then operates one or both of the high-side inlet solenoid valve  180  and low-side inlet solenoid valve  184  to open, fluidly connecting the joint line  186  to the high-side or low-side, respectively, of the air conditioning system. The controller  120  operates at least one of the discharge solenoid valves  192 ,  200  to open. Refrigerant then flows through the associated control orifice  196 ,  204 , through the system oil separator  220 , and is vented to atmosphere via the discharge passage  232 . 
     The mass flow rate through an orifice is defined as a change in mass over a specified time interval. During the venting of the refrigerant from the system, the mass of the refrigerant vented can be determined if the mass flow rate of the refrigerant leaving the system and the duration of the venting are both known. The controller  120  is configured to track the duration of the vent by utilizing the timer  360 . The controller  120  is configured use the vent duration to calculate the mass flow rate ({dot over (m)}), which is defined as the change in mass (m) over time (t). 
     
       
         
           
             
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                 Δ 
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                 ⁢ 
                 m 
               
               
                 Δ 
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                 t 
               
             
           
         
       
     
     If the pressure in the air conditioning system is supersonic, or greater than approximately 1.9 times the atmospheric pressure, flow through the orifice  196  or  204  is choked, or restricted. The mass flow rate can therefore be calculated using the choked orifice flow equation: 
               m   .     =     C   *   A   *       k   *   ρ   *         P   1     ⁡     (     2     k   +   1       )           k   +   1       k   -   1                     
where C is a discharge coefficient based on the type of flow through the orifice, A is the cross-sectional area of the orifice, k is the specific heat of the refrigerant, ρ is the density of the refrigerant, and P 1  is the upstream pressure, as measured by the pressure transducer  132 ,  148  corresponding to the inlet valve  180 ,  184 , respectively, that is open. When the pressure upstream of the orifice falls below 1.9 times the atmospheric pressure, the flow is no longer choked by the orifice, and the following subsonic mass flow equation is used to determine the mass flow rate:
 
               m   .     =     ρ   *   A   *         2   *     (       P   1     -     P   2       )       ρ               
where P 2  is the atmospheric pressure.
 
     Since the mass flow rate is equal to the change in mass (Δm) over the change in time (Δt), the change in mass is equal to the mass flow rate multiplied by the time elapsed.
 
Δ m={dot over (m)}*Δt  
 
Substituting the above flow equations into the change in mass equation, the change in mass during a venting operation can be calculated as:
 
               Δ   ⁢           ⁢   m     =       (     C   *   A   *       k   *   ρ   *         P   1     ⁡     (     2     k   +   1       )           k   +   1       k   -   1               )     *   Δ   ⁢           ⁢   t           
for supersonic flow through the orifice, and
 
               Δ   ⁢           ⁢   m     =       (     ρ   *   A   *         2   *     (       P   1     -     P   2       )       ρ         )     *   Δ   ⁢           ⁢   t           
for subsonic flow.
 
     For systems having multiple vent orifices, for example the system depicted in  FIG. 3  having two orifices  196 ,  204 , the mass flow through each individual orifice is calculated using the above equations, and the total mass vented from the system is the sum of the mass vented through each individual orifice. Any desired number of orifices having various diameters may therefore be used in the system to more precisely control the flow of refrigerant to the atmosphere. 
     In some embodiments, the mass flow rate is kept below a predetermined threshold, which may be approximately 100-140 grams per second in one embodiment, and which may be 120 grams per second in another specific embodiment, to prevent damage to the components and elastomeric seals of the air conditioning system as the system is vented. It is also advantageous, however, to keep the mass flow rate as close as possible to this predetermined maximum in order to vent the refrigerant from the system as quickly as possible. The solenoid valves corresponding to the orifices are therefore controlled to vent the refrigerant from the air conditioning system at a flow rate that is as close as possible to, without exceeding, the predetermined threshold. 
       FIG. 5  illustrates a method  400  of operating an embodiment of an ACS system, such as the ACS unit  100  described above with reference to  FIGS. 3 and 4 , during a venting operation. The processor  352  in the controller  120  is configured to execute programmed instructions stored in the memory  356  to operate the components in the ACS unit  100  to implement the method  400 . 
     The process  400  begins with the controller obtaining the pressure signal (block  404 ). The pressure signal is obtained from a pressure transducer upstream of the orifices. In the example of  FIG. 3 , the pressure signal is obtained from the pressure transducer  132  or  148  corresponding to the inlet valve  180  or  184 , respectively, that is open. Next, the controller determines whether the pressure upstream of the orifices is below a lower pressure threshold (block  408 ). 
     If the pressure is above the lower threshold, meaning that there is enough refrigerant remaining in the system for the venting operation to continue, then the controller proceeds to compute a theoretical mass flow through the orifices (block  412 ). The theoretical mass flow through the orifices is based on the pressure reading obtained upstream of the orifices and the supersonic and subsonic orifice flow equations discussed above. The ACS system may contain any number of orifices having a variety of different areas, and the theoretical mass flow calculation is performed for each of the orifices individually. The controller then determines the valves that should be opened to obtain the maximum flow of refrigerant out of the system without exceeding a maximum flow threshold (block  416 ). The controller determines which combination of discharge valves are to be opened to maximize the flow, and thus reduce the total time needed for venting the refrigerant, without exceeding the predetermined threshold at which the flow can cause damage to the components and elastomeric seals in the ACS system. Once the controller determines which valves to open for maximum desired flow, the controller proceeds to operate the selected valves to open (block  420 ) and the process continues at block  404 . 
     Once the pressure has dropped below the lower threshold (block  408 ), the flow through the orifices is essentially negligible, and the controller operates the valves to close (block  424 ). The venting operation is then complete (block  428 ). The process may then be initiated again for the other circuit, for example the low-side of the air conditioning system if the high-side was previously vented. 
       FIG. 6  illustrates a method  500  of tracking the total mass of refrigerant vented during a venting operation. The processor  352  is configured to execute programmed instructions stored in the memory  356  to operate the components in the ACS system  100  to implement the method  500 . The process  500  begins with the controller obtaining the pressure signal (block  504 ). The pressure signal is obtained from a pressure transducer upstream of the orifices (e.g. orifices  196  and  204 ). In the example of  FIG. 3 , the pressure signal is obtained from the pressure transducer  132  or  148  corresponding to the inlet valve  180  or  184 , respectively, that is open. 
     The processor then determines whether the flow is subsonic or supersonic (block  508 ). As discussed above, the flow is subsonic if the upstream pressure is less than approximately 1.9 times atmospheric pressure, while the flow is supersonic if the upstream pressure is greater than approximately 1.9 times atmospheric pressure. The controller then proceeds to compute the mass flow rate through the orifice (block  512 ) based on the mass flow rate equations discussed above. Next, the controller determines whether a predetermined time interval has elapsed using the timer associated with the controller (block  516 ). If the predetermined time interval has not elapsed, the process continues from block  504 . In one particular embodiment, the sampling rate is 0.2 seconds, and the predetermined time interval is one second, such that the blocks  504 - 516  are repeated five times before advancing to the next step. 
     Once the predetermined time interval has passed, the controller calculates the average mass flow rate over the predetermined time interval (block  520 ) based on the previously computed mass flow rates. The controller then determines the vented mass, which is the product of the average mass flow rate and the predetermined time interval. The controller stores the vented mass in the memory and adds the vented mass to a total mass vented variable, which is a running variable to which the vented mass is added at each cycle during the venting operation, in the memory (block  524 ). 
     The controller then proceeds to determine whether the valve is still open (block  528 ). If the valve is still open, the venting process is ongoing and the process then continues at block  504 . As discussed above, if the valve has been closed, the venting process has been terminated and the process for determining the mass vented ends (block  532 ). 
     In some embodiments, the controller is configured to determine the total mass vented without averaging the mass flow rate over a predetermined time. Instead of performing the steps in blocks  516  and  520 , the controller merely determines the mass flow rate of the refrigerant during the single sampling interval. The determined mass flow rate is then multiplied by the time between sampling intervals to obtain the mass vented, and the mass vented during the single sampling interval is added to the total mass vented variable. 
     In some embodiments, the processes described above with reference to  FIGS. 5 and 6  may be performed concurrently by the ACS system. In other embodiments, the system operation may be performed as in process  400 , while the mass determination is performed using a different process. In still other embodiments, the vented mass may be determined as in the method  500 , while the operation of the system is performed using a different process. 
       FIG. 7  illustrates a graph  600  of theoretical mass flow rate  604  versus time for a simulation of a venting process for venting carbon dioxide from an air conditioning system. The simulation assumed that a constant 10 psi was lost from the air conditioning system each second until no carbon dioxide remained in the system. As can be seen from  FIG. 7 , the mass flow rate  604  through the orifice decreases over time as the upstream pressure drops. 
       FIG. 8  illustrates a graph  620  of the total mass vented  624  over time for the same simulation as  FIG. 7 . As the mass flow rate decreases due to pressure decrease in the system, the slope, or rate of mass loss in the system, decreases. In the simulation illustrated in the graphs of  FIGS. 7 and 8 , approximately 30.75 grams of carbon dioxide was vented in about 78 seconds. 
     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.