Abstract:
A refrigerant charging system for charging a refrigeration system with refrigerant includes a refrigerant source, a storage vessel, input and output lines, and a device for detecting mass of refrigerant within the storage vessel. The input line fluidly connects the refrigerant source to the storage vessel, and the output line extends from the storage vessel and is adapted to connect to the refrigeration system. The system also includes an input control valve disposed between the storage vessel and the refrigerant source, and an output control valve disposed between the storage vessel and the refrigeration system. A heater is connected to the refrigerant source for raising the temperature of refrigerant within the refrigerant source. A method of charging a refrigeration system is also disclosed. The system and method are useful to charging the refrigerant system of an automotive vehicle, among others.

Description:
TECHNICAL FIELD 
     The disclosure relates generally to refrigerant charging systems and, more specifically, to a high precision refrigerant charging system using a constant volume tank. 
     BACKGROUND ART 
     Most refrigeration systems are not 100% free of leaks. Thus, the amount (or mass) of refrigerant within the refrigeration system decreases over time. Refrigeration systems, however, are designed to operate with a specific amount of refrigerant. Therefore, loss of refrigerant in a refrigeration system over time typically reduces the efficiency of the refrigeration system. Also, if the amount of refrigerant in the refrigeration system drops to a certain level, the refrigeration system may cease to operate and/or be damaged. For these reasons, a common maintenance operation for a refrigeration system is to recharge the refrigerant within the refrigeration system. 
     The recharging operation typically involves flushing the refrigeration system of any remaining refrigerant and, if present, other materials within the refrigeration system. Once the refrigeration system is flushed, a predetermined amount of new refrigerant is introduced into the refrigeration system. An important component of conventional refrigeration charging systems is the device that measures the amount of refrigerant introduced into the refrigeration system. Since the refrigeration system is designed to operate with a specific amount of refrigerant, too little or too much refrigerant can reduce the effectiveness of the recharging operation. 
     One conventional device used to measure the amount of refrigerant introduced into the refrigeration system is a load cell (or scale). Essentially, the load cell measures the weight of a tank containing the refrigerant before the refrigerant is introduced into the refrigeration system and then afterwards. The difference between the two readings is the amount of refrigerant introduced into the refrigeration system. There are, however, certain problems that are associated with the use of a load cell in conventional recharging systems. Load cells are sensitive to vibration, which can throw off the measurements. Also, since the load cell determines the weight of the entire tank used to supply the refrigerant, which can weigh several hundred pounds or more, sensitivity of the load cell is reduced. 
     Furthermore, recent advances in refrigeration technology employ carbon dioxide as the refrigerant, which is stored at a pressure as much as ten times higher than the pressure at which conventional refrigerants are stored, and the resulting increase in storage pressure necessitates thicker walls for the tank. These thicker walls add additional weight to the tank, which further decreases the sensitivity of the load cell. 
     Another conventional device used to measure the amount of refrigerant introduced into the refrigeration system employs mass flow technology. As recognized by those skilled in the art, mass flow technology implements a sensor that measures the flow rate of fluid (i.e., the refrigerant) flowing past a certain point. However, use of mass flow technology is very expensive, and the expense is even greater when an increased sensitivity for calculating the amount of refrigerant delivered is desired. The accuracy of mass flow technology is dependent on the fluid state since the mass flow sensors do not measure gas as well as liquid. Although most refrigerants are in a liquid form during the recharging operation, carbon dioxide is in a gaseous state during a recharging operation. Also, mass flow technology does not work well with two-phase fluids. There is, therefore, a need for an refrigerant charging system and method that is more accurate and vibration-resistant, and less expensive than conventional refrigeration charging systems. 
     SUMMARY OF THE DISCLOSURE 
     Described is a system for charging a refrigeration system with refrigerant. The charging system includes a refrigerant source, a storage vessel, input and output lines, and a device for detecting mass of refrigerant within the storage vessel. The input line fluidly connects the refrigerant source to the storage vessel, and the output line extends from the storage vessel and is adapted to connect to the refrigeration system. The recharging system may also include an input control valve disposed between the storage vessel and the refrigerant source, and an output control valve disposed between the storage vessel and the refrigeration system. A heater may optionally be connected to the refrigerant source for raising the temperature of refrigerant within the refrigerant source. 
     The refrigeration system is charged with refrigerant by determining mass of refrigerant needed to charge the refrigeration system and transferring refrigerant from a refrigerant source to a storage vessel. An initial mass of refrigerant in the storage vessel is determined after the refrigerant is transferred to the storage vessel. The refrigerant is then transferred from the storage vessel to the refrigeration system, and the mass of refrigerant transferred to the refrigeration system is determined. The determined mass of refrigerant transferred to the refrigeration system is compared with the determined mass of refrigerant for charging. If the total mass of refrigerant transferred to the refrigeration system does not equal the determined mass of refrigerant for charging, the steps of transferring refrigerant from the refrigerant source to the storage vessel and transferring refrigerant from the storage vessel to the refrigeration system are repeated. 
     The mass of refrigerant transferred to the refrigeration system is calculated by comparing the initial mass with a remaining mass of refrigerant within the storage vessel, which is determined based on temperature and pressure of the refrigerant in the storage vessel. While determining the mass of refrigerant within the storage vessel, the volume of the storage vessel remains constant. The storage vessel is fluidly disconnected from the refrigeration system prior to determining the remaining mass of refrigerant, and the refrigerant source is fluidly disconnected from the storage vessel prior to determining the initial mass of refrigerant. 
     Additional advantages will become readily apparent to those skilled in the art from the following detailed description, wherein only an exemplary embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is made to the attached drawings, wherein elements having the same reference numeral designations represent like elements throughout, and wherein: 
         FIG. 1  is a schematic view of a refrigerant charging system, according to the disclosure; and 
         FIG. 2  is a flow chart of a method of charging a refrigeration system, according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An example of a refrigerant charging system  10  according to the disclosure is illustrated in  FIG. 1 . The refrigerant charging system includes a refrigerant source  12 , a storage vessel  14 , an input line  16 , and an output line  18 . The input line  16  fluidly connects the refrigerant source  12  to the storage vessel  14 , and the input line  16  includes an input control valve  22  for allowing refrigerant to flow to/from the input line  16  from/to the storage vessel  14 . The output line  18  fluidly connects the storage vessel  14  to a refrigeration system (not shown) to be charged of a vehicle  20 , and the output line  18  includes an output control valve  24  for allowing refrigerant to flow to/from the storage vessel  18  from/to the refrigeration system of the vehicle  20 . 
     The storage vessel  14  can also include a temperature sensor  26  and a pressure sensor  28  for measuring the temperature and pressure of the refrigerant within the storage vessel. The use of a temperature and pressure sensors  26 ,  28  to measure temperature and pressure of a fluid within a vessel is well known in the art, and the refrigerant charging system  10  is not limited as to any particular types or configurations of temperature and pressure sensors  26 ,  28 . 
     The storage vessel  14  is not limited as to a particular size. However, reducing the size of the storage vessel  14  allows for greater sensitivity in charging the refrigeration system of the vehicle. As will be discussed in more detail below, the method of charging the refrigeration system can involve multiple occurrences of the refrigerant in the storage vessel  14  being discharged into the refrigeration system to be charged. By reducing the size of the storage vessel  14 , the amount of refrigerant discharged during a particular cycle is reduced, which increases sensitivity. However, by reducing the size of the storage vessel  14 , the number of cycles needed to fully charge the refrigeration system increases, which disadvantageously increases the amount of time to charge the refrigeration system. 
     The refrigerant source  12  is not limited as to a particular type of source for providing refrigerant. However, in a current aspect of the refrigerant charging system  10 , the refrigerant source  12  is a constant-volume tank. The tank  12  of refrigerant can also include a heater  30  for adjusting the temperature of refrigerant within the tank  12 . The use of a heater  30  to adjust the temperature of a fluid within a tank is well known in the art, and the present refrigerant charging system  10  is not limited as to any particular type or configuration of heater  30 . However, in a current aspect of the refrigerant charging system  10 , the heater  30  is an electrically-adjusted resistance band that can be wrapped around the tank  12 . 
     The refrigerant charging system  10  can optionally include a controller  32  connected to at least one of the sensors  26 ,  28 , control valves  22 ,  24  and heater  30 . Although the control valves  22 ,  24  and the heater  30  can be operated manually, the controller  32  can be used to automatically control the operation of these devices. Additionally, information from the sensors  26 ,  28  can be sent to the controller  32 , either automatically or manually, and the controller  32  can use this information during the control of the control valves  22 ,  24  and heater  30 . 
     The operations of the refrigerant charging system  10  are schematically illustrated in the flow chart of  FIG. 2 . The mass of new refrigerant to be introduced into the refrigeration system is predefined by the manufacturer of the refrigeration system. In an initial step  110 , the initial mass in the storage vessel  14  is determined. The initial mass consists of the required amount (mass) of refrigerant to be charged into the refrigeration system of the vehicle plus the expected remaining refrigerant using the following equation:
 
Initial Mass=Required Mass+Expected Remaining Mass  (1)
 
     Although other techniques can be used to determine the expected remaining mass and the required mass of refrigerant to be charged into the refrigeration system of the vehicle, in a current aspect of the methodology of charging a refrigeration system, the expected remaining mass for the first charge is set to be equal to the required mass and the required mass of refrigerant is determined by using the temperature and pressure readings respectively from the temperature and pressure sensors  26 ,  28  in the following equation:
 
Required Mass=Density×Volume  (2)
 
Density=[Pressure×Molecular Mass]/[Gas Constant×Temperature]  (3)
 
     Density is a function of pressure and temperature with a known molecular mass (M) for specific refrigerant and Universal Gas Constant (R=8.314 (kJ/kmol ° K)). The calculation of density and initial mass can be done manually or by the controller  32 . The volume used in for the calculation includes the volume of the storage vessel  14 , the volume of the input line  16  between the input control valve  22  and the storage vessel  14 , and the volume of the output line  18  between the storage vessel  14 , and the output control valve  24 . 
     In step  120 , a mass of refrigerant is introduced from the refrigerant source  12  into the storage vessel  14  via the input line  16  by opening the input control valve  22 . Based on the required refrigerant in the storage vessel  14  determined on step  110 , the amount (mass) of refrigerant in the storage vessel is adjusted using the readings from the temperature and pressure sensors  26 ,  28  respectively during the fluid transfer. Although the pressure of the refrigerant in the refrigerant source  12  and the refrigerant in the storage vessel  14  need not be equalized prior closing the input control valve  22 , in a current aspect of the methodology of charging a refrigeration system, the pressure of the refrigerant in the refrigerant source  12  and the refrigerant in the storage vessel  14  is equalized prior closing the input control valve  22 . The closing of the input control valve  22  isolates the storage vessel  14  from the refrigerant source  12 . 
     Once the initial mass of refrigerant in the storage vessel is transferred and isolated from the refrigerant source  12 . In step  130 , the initial mass on the storage tank can be verified using the equation 1, 2, and 3 to ensure accuracy. The output control valve  24  is then opened and refrigerant is allowed to flow from the storage vessel  14  through the output line  18  and into the refrigeration system of the vehicle  20  in step  140 . The pressure of the refrigerant in the refrigeration system of the vehicle  20  and the refrigerant in the storage vessel  14  is allowed to equalize, and the output control valve  24  is then closed. The closing of the output control valve  24  isolates the storage vessel  14  from the refrigeration system of the vehicle  20 . 
     After the storage vessel  14  is isolated from the refrigeration system of the vehicle  20 , the actual remaining mass of refrigerant in the storage vessel  14  is determined in step  150 . Although other techniques can be used to determine the mass of refrigerant in the storage vessel  14 , in a current aspect of the methodology of charging a refrigeration system, the actual remaining mass of refrigerant is determined by using the temperature and pressure readings respectively from the temperature and pressure sensors  26 ,  28  in equation 3 and the following equation:
 
Actual Remaining Mass=Density×Volume  (4)
 
     After the actual remaining mass of refrigerant in the storage vessel  14  is calculated, a determination is made as to whether or not mass needs to be added or removed from the refrigeration system in the vehicle  20  in step  160 . This involves comparing the required mass of refrigerant to be charged based on refrigeration system manufacturer specification with the amount of refrigerant actually introduced into the refrigeration system of the vehicle  20 . The amount of refrigerant actually introduced into the refrigeration system of the vehicle  20  is calculated using the equation:
 
Mass Introduced=Initial Mass−Actual Remaining Mass  (5)
 
     If the mass of refrigerant introduced into the refrigeration system equals the required mass +/− a predetermined deviation, the output line  18  can be disconnected from the refrigeration system of the vehicle  20 . In certain situations when the actual mass of refrigerant introduced into the refrigeration system is less than the required mass of refrigeration, then starting from the second charge forward, the expected remaining mass on equation 1 is set to be the actual remaining mass from the previous charge in step  170  and steps  110  through  160  are repeated until the total mass of refrigerant introduced into the refrigeration system equals the required mass +/− the predetermined deviation. 
     When step  120  is repeated, the heater  30  connected to the refrigerant source  12  can be used to increase the temperature of the refrigerant entering into the storage vessel  14 . This enables the refrigerant in the refrigerant source to be at a higher pressure than the remaining refrigerant in the storage vessel  14  and creates a flow of refrigerant from the refrigerant source  12  to the storage vessel  14 . The transfer will result in an increase in pressure of the refrigerant in the storage vessel  14 . When the pressure of the refrigerant in the storage vessel  14  is increased, the mass of refrigerant in the storage vessel  14  also increases, which enables additional mass to be introduced into the refrigeration system during step  140 . 
     Use of the controller  32  allows for calculation of the initial mass, remaining mass, and introduced mass. The controller  32  can also operate the valves  22 ,  24  and heater  30  in a manner in which the pressure of the refrigerant being introduced into the storage vessel  14  can be closely controlled. In doing so, a need to repeat steps  110  through  160  can be minimized. 
     Through use of the present refrigerant charging system, refrigerant can be charged into a refrigeration system with accuracy comparable to or improved compared to prior technology used for the same purpose. Also, since the present refrigerant charging system can function without mass flow technology, the present refrigerant charging system is less expensive than those systems that employ mass flow technology and has increased accuracy, such as with carbon dioxide refrigerants. Furthermore, since the present refrigerant charging system does not employ a load cell, the system is less sensitive to vibration, which has been a source of inaccuracy with prior systems that employ a load cell. 
     The disclosed concepts may be practiced by employing conventional methodology and equipment. Accordingly, the details of such equipment and methodology are not set forth herein in detail. In the previous descriptions, numerous specific details are set forth, such as specific formulas, processes, techniques, etc., in order to provide a thorough understanding of the present invention. However, it should be recognized that the present invention may be practiced without resorting to the details specifically set forth. 
     Only an exemplary aspect of the present disclosure and but a few examples of its versatility are shown and described. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein.