Abstract:
A refrigerant flow metering device for use in a refrigeration system which uses a movable piston with a fixed flow passage in series with a variable area flow passage. The variable flow area is defined by the position of the piston relative to the containing housing. The position of the movable piston is a function of refrigerant pressure differential across the piston and the force of a spring member acting in an opposite direction on the piston. The variable flow area is defined as the clearance between a tapered fixed housing and the tapered portion of the movable piston. The piston is sealed via an “O” ring seal causing fluid to flow first through a fixed bore and then through the variable flow area.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 09/512,177, filed on Feb. 24, 2000. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to variable flow metering devices and more particularly to variable flow area refrigerant expansion devices for use in controlling compression refrigeration systems so as to be responsive to a pressure differential between high and low pressure areas of a refrigeration system. 
     BACKGROUND OF THE INVENTION 
     Most small commercial vehicles and particularly automotive and mobile air conditioning systems, as represented in FIG. 1, utilize a fixed area refrigerant expansion device for flow control to reduce or eliminate moving parts and reduce costs. Known fixed area expansion devices are long capillary tubes or relatively short orifice tubes in the high-pressure liquid line between the condenser and evaporator. The refrigerant flow is primarily dependent on the inlet pressure (head pressure) at the orifice and the amount of liquid subcooling. In fixed area expansion devices, increasing head pressure or subcooling increases flow while increasing gas at the inlet decreases flow. Suction pressure changes have no effect on flow in normal system operation at higher ambient since flow is at “sonic” velocity and suction pressure is usually below the pressure at which this sonic flow occurs. As described in detail in my U.S. Pat. No. 5,901,750, incorporated herein by reference, these characteristics make the fixed area expansion device or capillary tube self regulating and produces adequate performance in a wide range of conditions encountered in normal automotive air conditioner operation with the exception of performance at idle. In automotive use the fixed orifice tube is sized for high ambient load at high speed vehicle operation and is generally about twice as large at idle as is optimal, resulting in increased compressor horsepower and a reduction in cooling. FIG. 2 depicts a performance chart of a typical automotive system using a fixed orifice tube versus that of smaller orifices at idle. 
     Another known expansion device for use in flow control in refrigerant systems is a thermostatic expansion (TXV) valve. A TXV valve is a variable area expansion device and operates to control the flow rate of liquid refrigerant entering the evaporator as a function of the temperature of the refrigerant gas leaving the evaporator. However, while fixed area devices cannot match the efficiency of the TXV except at certain defined operating conditions, TXV valves are more complicated to manufacture and thus, more costly than the fixed devices. 
     An example of an automotive refrigerant system operation at high vehicle operation speed as compared to vehicle idle is as follows: At high operation speeds the head pressure may be approximately 250 PSIG and the sub-cooling 20° F. with a system capacity of approximately 24,000 BTU/Hr. At idle the head pressure of the vehicle rises to 350 PSIG due to reduction of condenser air flow and re-circulation of hot under-hood air into the condenser. Accordingly, balance is achieved at idle by reducing sub-cooling at the expansion device inlet. Initially, the rise in head pressure increases flow through the orifice tube. The result is that the sub-cooled liquid in the condenser is flushed out and uncondensed gas enters the orifice tube inlet. Gas greatly reduces flow until the flow rate out of the compressor is equal to flow through the orifice tube. Since capacity per pound of refrigerant is a function of liquid percentage after expansion, a gaseous mixture entering the expansion device greatly reduces capacity and system efficiency. At idle this capacity reduces to 12,000 BTU/Hr. in this example. If a smaller flow area is utilized in the expansion device refrigerant flow is reduced allowing liquid backup in the condenser and thus sub-cooling to occur. The net result is more cooling at reduced compressor work. 
     However, when small orifice arc used at high load high speed operational conditions, more refrigerant is required in the system as more back up of liquid occurs until enough subcooling and head pressure are available to again flow enough to satisfy evaporator requirements. Unfortunately, this causes the head pressure to rise to the range of 300-400 PSIG, substantially reducing the compressor life. Accordingly, the variable flow orifice must be adequate in flow area at high speed high load conditions, in order to operate satisfactory. Further, a very short orifice tube relative to it&#39;s diameter becomes more restrictive as sub-cooling is increased. This characteristic of the short orifice tube is a detriment in automotive use since flow starvation may occur at high vehicle operational speeds if the small orifice size is engaged. Starvation results in high compressor discharge superheat temperatures, which may be very detrimental to compressor durability. 
     An object of this invention is to provide a variable flow area valve sensitive to system operating pressure. 
     Another object of this invention is to provide a metering valve, which closely matches the flow characteristics of a capillary or orifice tube as opposed to a short orifice plate. 
     Another object of this invention is to isolate the spring from refrigerant flow for reasons of vibration, durability, and noise. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a variable flow area refrigerant expansion device includes a valve body having an axially extending aperture therethrough, a piston member slidably received in the valve body aperture, and a spring member operatively connected to the piston to resiliently urge the piston in a predetermined direction in the valve body aperture to insure fluid flow through the expansion device. The valve body aperture includes an inlet end and an outlet end. In accordance with one aspect of the invention, the interior surface of the valve body aperture at the outlet end is tapered from a first diameter to a second diameter, wherein the first diameter is larger than the second diameter, and terminates in a bore. The bore is a preferably a cross-bore. 
     The piston includes a flow passageway partially extending therethrough, with an inlet end and an outlet end. The outlet end terminates in a cross-bore formed though the piston body. In accordance with the invention, the piston further includes a tapered portion positioned downstream from the direction of fluid flow through the expansion device. The tapered portion has a first end defined by a first diameter and a second end defined by a second diameter, where the first diameter is larger than the second diameter. The piston is positioned within the valve body aperture with the tapered portion of the piston being received in the tapered outlet end of the valve body. In a preferred embodiment the tapered portion of the piston is tapered to a first predetermined angle and the tapered outlet end of the valve body is tapered to a second predetermined angle such that the tapers are not parallel to provide a mechanism for metering fluid through the expansion valve in response to pressure differentials between the inlet and outlet ends of the valve body. 
     In one embodiment, the spring member is disposed around the outer surface of the piston and contacts an annular wall within the valve body to resiliently urge the spring into a full open position. Alternatively, the valve body may be formed with a spring chamber positioned downstream of a distal end of the piston, such that an end of the spring engages the distal end of the piston to resiliently urge the spring into a full open position. 
     In accordance with another aspect of the invention, to insure that fluid flows through the passageway in the piston, it is preferred that the device include at least one valve body seal and at least one piston seal member. An outer surface of the valve body is preferably formed with an annular groove to receive a valve body seal, such as an O-ring. A second annular groove and valve body seal are preferably provided as a back-up seal, downstream of the first annular groove. 
     The interior surface of the valve body inlet end is preferably provided an annular surface for engaging piston seal member. In a preferred embodiment, the expansion device further includes a screen member that is connected to the inlet end of the valve body. A distal end of the screen member cooperates with the annular shoulder provided on the interior surface of the valve body such that the piston seal member is captured therebetween, thereby reducing the requirement for rigid tolerances to adequately retain piston seal member. Alternatively, the interior surface of the valve body may include an interior annular groove for retaining the piston seal member, or the inlet end of the piston may be provided with an annular groove for receiving the piston seal member. 
     In accordance with another aspect of the invention, it is preferred that a screen member is connected to the valve body by a snap-fit connection. Accordingly, the screen member is preferably provided with outwardly extending tang members that engage recesses formed in the interior surface of the valve body inlet end. Alternatively, the screen member may be molded onto the inlet end of the valve body or threadedly connected thereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and inventive aspects of the present invention will become more apparent upon reading the following detailed description, claims, and drawings, of which the following is a brief description: 
     FIG. 1 is a schematic diagram of a compression refrigeration system utilizing a variable flow area refrigerant expansion device in accordance with the present invention. 
     FIG. 2 is a graphical representation of automotive system cooling performance vs. orifice size at idle conditions. 
     FIG. 3 is a side view of the variable flow area refrigerant expansion device in accordance with the present invention. 
     FIG. 4 is a side cross sectional view of the variable flow area refrigerant expansion device in accordance with the present invention. 
     FIG. 5 is an end view of the variable flow area refrigerant expansion device shown in FIG. 4 from a screen entrance end portion. 
     FIG. 6 is a graphical representation of a variable flow area refrigerant expansion device orifice size vs. head pressure. 
     FIG. 7 is a side cross sectional view of an alternative embodiment of a variable flow refrigerant expansion device. 
     FIG. 8 is a section view of a valve body of the embodiment shown in FIG.  7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 2, a compression refrigeration system  10  is shown. System  10  is typical of that found in many air conditioning applications especially in automotive use. System  10  includes a compressor  12 , a condenser  14 , an expansion device  16 , and an evaporator  18 . In a preferred embodiment, system  10  further includes an optional suction accumulator  20 . Suction accumulator  20  is desirable in highly variable air conditioning load applications for storing refrigerant, to prevent compressor slugging and to allow refrigerant to be displaced from low pressure and high pressure areas. The components of system  10  are connected together by refrigerant lines to form a refrigerant circuit. In operation, compressor  12  is driven by a vehicle engine (not shown) and compresses refrigerant vapor to a temperature above the ambient condenser air temperature. Refrigerant is delivered from compressor  12  as high pressure vapor to an inlet of condenser  14  by a compressor discharge line (not shown). The condenser  14  removes heat from this vapor in a conventional manner and condenses all or most of this vapor to a liquid. This high-pressure refrigerant then is discharged as high pressure refrigerant liquid to a high-pressure liquid line  22  that is fluidly connected to the evaporator  18 . The expansion device  16  of the present invention is positioned within liquid line  22  between condenser  14  and evaporator  18 . 
     Once fluid flows to expansion device  16 , it is expanded to a low-pressure mixture of liquid and gas. This mixture flows to evaporator  18  where most or the entire liquid portion is evaporated as heat is absorbed from an external heat source such as relatively warm humid air. In automotive applications the refrigerant vapor along with oil and a small percentage of liquid flows then to suction accumulator  20  where all of the vapor and again a small percentage of liquid is metered back to compressor  12  where the cycle is repeated. 
     Referring to FIGS. 3-5, expansion device  16  will be described in greater detail. Expansion device  16  consists of a valve body  24 , a screen assembly  26 , a piston  28 , piston sealing member  30 , spring member  31  and at least one expansion device sealing member  32 . The device is shown positioned in line  22 , wherein line  22  further includes crimped portions  34  that extend radially inwardly to keep expansion device  16  in place within line  22 . 
     Screen assembly  26  is positioned at and operatively connected to an inlet end  36  of valve body  24  by means of an annular snap-fit end portion  38 . End portion  38  includes outwardly extending tang members  40  that engage an annular groove  42  formed within an internal surface  44  of valve body  24 . While a snap-fit connection is preferred to operatively connect screen assembly  26  to valve body  24 , other suitable connection members may be employed, Such as threaded connections. 
     Valve body  24  is generally hollow with a cylindrical outer surface  44  and having stepped sections with varying outer diameters. The first section  46  at inlet end  36  of valve body  24  has the largest diameter D 1  that is sized to closely fit within line  22  and is open to receive piston  28 , as explained in further detail below. First section  46  terminates at annular surface  48  which abuts against crimped portions  34  to retain valve body  24  within line  22 . Outer surface  44  of first section  46  includes at least one annular groove  50  formed therein to receive expansion device sealing member  32 . Preferably, sealing member  32  is an O-ring, although other suitable sealing members may be employed. To insure against refrigerant flowing around expansion device  16  in the event of a failure of sealing member  32 , in the preferred embodiment, there is a second expansion device sealing member  32   a  positioned within a second annular groove  50   a  downstream of sealing member  32 . In one preferred embodiment, a distal end  52  of valve body  24  has a cross-bore  54  formed therethrough to permit fluid to flow through expansion device  16 , to be explained in further detail below. Alternatively, distal end  52  may be formed with a through bore  55 , represented in phantom in FIG.  4 . 
     Piston  28  is movably positioned within valve body  24 . Piston  28  has a first cylindrical end portion  56 , a main body section  58 , an outlet portion  60  and a tapered second end portion  62 . Extending partially through piston end portion  56 , main body section  58  and terminating at outlet portion  60  is a flow passageway  64 . Passageway  64  has an inlet  65  positioned adjacent to screen assembly  26  to permit refrigerant to flow through expansion valve assembly  16 . Main body section  58  has a diameter d 2  that is smaller than the diameter of first cylindrical end portion  56  and receives spring member  31  to counteract piston  28  movement due to pressure differential across the piston  28 . Spring member  31  is preferably a calibrated compression spring although other suitable spring members may be used. An end portion  68  of main body section  58  tapers inwardly and terminates in a cross-bore  70 . Adjacent to and downstream of cross-bore  70  is tapered second end portion  62 . Tapered second end portion  62  has an outer surface  72  that is preferably tapered to a first predetermined angle and terminates at a stop surface  74 . 
     In accordance with one aspect of the invention, when piston  28  is positioned within valve body  24 , tapered second end portion  62  is positioned within a flow chamber  76  defined by an interior surface  78  of valve body  24 . Interior surface  78  has a tapered portion  79  that cooperates with outer surface  72  of tapered second end portion  62  of piston  28  to meter fluid through expansion valve assembly  16 . Preferably, interior surface  79  is tapered at a second predetermined angle so as to provide a sufficiently long orifice tube length when piston  28  is moved in its full open position, downstream, with stop surface  74  abutting stop members  80  formed on the interior surface of distal end  52 . 
     In accordance with another aspect of the invention, to insure that fluid flows though flow passageway  64 , expansion device  16  is provided with piston sealing member  30 . In a preferred embodiment, sealing member  30  is an O-ring and is positioned around first cylindrical end portion  56 . End portion  38  of screen assembly  26  cooperates with an annular surface  48  to retain sealing member  30  on piston  28  and to insure that refrigerant is forced to flow though passageway  64  without requiring machining of an annular ring on an outer surface  81  of first cylindrical end portion  56 . 
     During system operation, piston  28  is positioned in an open position, with piston  28  being displaced fully leftward such that stop surface  74  of piston  28  is spaced away from stop members  80 . Refrigerant flows through screen assembly  26  as shown by flow arrows in FIGS. 3-4, and is forced to enter flow passageway  64  due to piston sealing member  30 . Once within flow passageway  64 , refrigerant exits piston  28  through cross-bore  70 , located upstream of tapered second end portion  62 . Refrigerant then flows between tapered second end portion  62  and tapered interior surface  79 , metering refrigerant whereby refrigerant exits valve body  28  through cross-bore  54 . In accordance with the invention, as refrigerant exits through cross-bore  54 , piston  28  begins to move when a predetermined pressure differential acts on piston  28 , moving piston rightward such that stop surface  74  of piston moves toward stop member  80 . As piston  28  moves rightward within valve body  28 , the clearance between tapered second end portion  62  and interior surface  79  decreases. As piston  28  moves rightward against spring member  66 , an increase in pressure occurs at chamber  76 . This results in a very gradual movement as function of pressure differential across expansion device  16  as piston portion  56  has a decreasing pressure differential in comparison to a smaller tapered  62  cross-section increasing pressure differential. Further, the pressure differential across the piston inlet  65  and chamber  76  varies as a function of refrigerant inlet state—i.e. quality or subcooling. If, when there is equal inlet pressures, vapor (quality) is present in piston inlet refrigerant then pressure will be substantially lower at cross-bore  74  at the outlet of passageway  64  than if subcooled liquid is flowing. With vapor present at idle conditions, expansion device  16  will actuate at substantially lower head pressure as compared to a subcooled refrigerant state, which would exist in high speed vehicle conditions. This results in a large modulation range of orifice size of the expansion device as a function of head pressure at constant suction pressure, as shown in FIG.  6 . The cooperating surfaces of tapered second end portion  62  and interior surface  79  are designed to essentially have equal inlet and outlet flow areas at a full rightward position resulting in an adequately long orifice tube length. Accordingly, this invention is responsive to high to low side pressure differential across expansion device  16 . 
     FIGS. 7 and 8 depict and an alternative embodiment of an expansion device  116 . Similar to expansion device  16 , expansion device  116  includes a valve body  124 , screen assembly  126 , a piston  128 , piston sealing member  130 , a spring member  131  and at least one expansion device sealing member  132 . Screen assembly  126  is positioned at and operatively connected to an inlet end  136  of valve body  124  by means of an annular snap-fit end portion  138 . End portion  138  includes outwardly extending tang members  140  that engage an annular groove  142  formed within an interior surface  143  of valve body  124 . 
     Valve body  124  is generally hollow with a cylindrical outer surface  144  and having stepped sections with varying outer diameters. The first section  146  at inlet end  136  of valve body  124  has the largest diameter D, that is sized to closely fit within line  22  and is open to receive piston  128 , as explained in further detail below. First section  146  terminates at annular surface  148  which abuts against crimped portions  34  to retain valve body  124  within line  22 . Outer surface  144  of first section  146  includes at least one annular groove  150  formed therein to receive expansion device sealing member  132 . Preferably, sealing member  132  is an O-ring, although other suitable sealing members may be employed. To insure against refrigerant flowing around expansion device  116  in the event of a failure of sealing member  32 , in the preferred embodiment, there is a second expansion device sealing member  132   a  positioned within a second annular groove  150   a  downstream of sealing member  132 . A distal end portion  152  of valve body  124  has a second diameter D 2  that is smaller than diameter D 1 , has a cross-bore  154  formed therethrough, to be explained in further detail below. Alternatively, distal end  152  may be formed with a through bore  155 , represented in phantom in FIG.  7 . 
     Piston  128  is movably positioned within valve body  124 . Piston  128  has a first cylindrical inlet end portion  156 , an outlet portion  160  and an elongated tapered second end portion  162 . Extending through piston end portion  156  and terminating at outlet portion  160  is a flow passageway  164  that opens into a cross-bore  170 . Passageway  164  has an inlet  165  positioned adjacent to screen assembly  126  to permit refrigerant to flow through expansion valve assembly  116 . Adjacent to and downstream of cross-bore  170  is elongated tapered second end portion  162 . Elongated tapered second end portion  162  has a first outer surface  172  that is preferably tapered to a first predetermined angle that transitions to a cylindrical end portion  174 . Cylindrical end portion  174  has an integral extension member  175  that serves as a stop surface. A spring chamber  176  is positioned within valve body  124 , which receives spring member  131  to counteract piston  128  movement due to pressure differential across the piston  128 . An integral extension member  177  extends inwardly into spring chamber  176  from a distal end  155  of valve body  124  and serves as a stop member that cooperates with integral extension member  175  of tapered second end portion  162 . Spring member  131  is positioned within spring chamber  176  on integral extension members  175  and  177 . Spring member  131  is preferably a calibrated compression spring although other suitable spring members may be used. To permit debris and dirt to escape from spring chamber  176 , an interior surface  179  of valve body  124  is provided with key members  181  of elongated tapered second end portion  162 . 
     In accordance with one aspect of the invention, when piston  128  is positioned within valve body  124 , elongated tapered second end portion  162  is positioned within a flow chamber  180  defined by an interior surface  178  of valve body  124 . Interior surface  178  has a tapered portion  183  that cooperates with outer surface  172  of elongated tapered second end portion  162  of piston  128  to meter fluid through expansion valve assembly  116 . Preferably, interior surface  178  is tapered at a second predetermined angle so as to provide a sufficiently long orifice tube length when piston  128  is moved in its full open position, downstream, with extension members  175  and  177  abutting one another. 
     In accordance with another aspect of the invention, to insure that fluid flows though flow passageway  164 , expansion device  16  is provided with piston scaling member  130 . In a preferred embodiment, sealing member  130  is an O-ring and is positioned around first cylindrical end portion  156 . End portion  138  of screen assembly  126  cooperates with an annular surface  148  to retain sealing member  130  on piston  128  and to insure that refrigerant is forced to flow though passageway  164  without requiring machining of an annular ring on an outer surface  183  of first cylindrical end portion  156 . 
     During system operation, piston  128  is positioned in an open position, with piston  128  being displaced fully leftward such that extension members  175  and  177  are spaced away from one another. Refrigerant flows through screen assembly  126  as shown by flow arrows in FIGS. 7-8, and is forced to enter flow passageway  164  due to piston sealing member  130 . Once within flow passageway  164 , refrigerant exits piston  128  through cross-bore  170 , located upstream of tapered second end portion  162 . Refrigerant then flows between tapered second end portion  162  and tapered interior surface  183 , metering refrigerant whereby refrigerant exits valve body  128  through cross-bore  154 . In accordance with the invention, as refrigerant exits through cross-bore  154 , piston  128  begins to move when a predetermined pressure differential acts on piston  128 , moving piston rightward such that integral extension member  175  of piston  128  moves toward extension member  177 . As piston  128  moves rightward within valve body  124 , the clearance between tapered second end portion  162  and interior surface  183  decreases. As piston  128  moves rightward against spring member  131 , an increase in pressure occurs at flow passageway  180 . This results in a very gradual movement as function of pressure differential across expansion device  116  as flow chamber  180  has a decreasing pressure differential in comparison to a smaller tapered cross-section increasing pressure differential. This results in a large modulation range of orifice size of the expansion device as a function of head pressure at constant suction pressure, as previously discussed in connection with FIG.  6 . The cooperating surfaces of tapered second end portion  162  and interior surface  183  are designed to essentially have equal inlet and outlet flow areas at a full rightward position resulting in an adequately long orifice tube length. Accordingly, this invention is responsive to high to low side pressure differential across expansion device  116 . 
     Although certain preferred embodiments of the present invention have been described, the invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention. A person of ordinary skill in the art will realize that certain modifications and variations will come within the teachings of this invention and that such variations and modifications are within its spirit and the scope as defined by the claims.