Abstract:
An improved method of reducing wear dust and torque required to turn a multi-port rotary disc valve utilizes a thrust bearing to hold the valve seat and/or valve disc such that they are not in contact with each other, or have light contact each other.

Description:
This application is a National Phase Application of International Application No. PCT/US2005/007981, filed Mar. 8, 2005, which claims the benefit under 35 U.S.C. 119 (a-e) of U.S. Provisional Application No. 60/551,154 filed Mar. 8, 2004, which is herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to cryogenic refrigerators, in particular, Gifford McMahon (GM) refrigerators, GM type pulse tube refrigerators, and Solvay refrigerators. Coldheads of such cryogenic refrigerators include a valve mechanism, which commonly consists of a rotary valve disc and a valve seat. There are discrete ports, which, by periodic alignment of the different ports, allow the passage of a working fluid, supplied by a compressor, to and from the regenerators and working volumes of the coldhead. 
   GM and Solvay type refrigerators use compressors that supply gas at a nearly constant high pressure and receive gas at a nearly constant low pressure. The gas is supplied to a reciprocating expander that runs at a low speed relative to the compressor by virtue of a valve mechanism that alternately lets gas in and out of the expander. 
   W. E. Gifford also conceived of an expander that replaced the solid displacer with a gas displacer and called it a “pulse tube” refrigerator. This was first described in his U.S. Pat. No. 3,237,421 which shows a pulse tube connected to valves like the earlier GM refrigerators. 
   Early pulse tube refrigerators were not efficient enough to compete with GM type refrigerators. A significant improvement was made by Mikulin et al., as reported in 1984, and significant interest ensued in looking for further improvements. Descriptions of major improvements since 1984 can be found in the references listed herein. All of these pulse tubes can run as GM type expanders that use valves to cycle gas in and out of the pulse tube. GM type pulse tubes running at low speed are typically used for applications below about 20 K. It has been found that best performance at 4 K has been obtained with the pulse tube shown in FIG. 9 of U.S. Pat. No. 6,256,998. This design has six valves which open and close in the sequence shown in  FIG. 11 . 
   U.S. Pat. No. 3,119,237 by W. E. Gifford shows an early pneumatically driven GM expander and a multi-ported rotary spool valve to control gas flow to the regenerator out of phase with gas flow to the drive piston. In a subsequent U.S. Pat. No. 3,205,668, Gifford discloses a multi-ported rotary disc valve that uses the high to low pressure difference to maintain a tight seal across the face of the valve. He states that this type of valve is superior to the spool type valve because the leak rate is lower, even after it has run a long time and has experienced some wear. This type of valve has been widely used in different types of GM refrigerators as shown for example in U.S. Pat. Nos. 3,620,029, 3,625,015, 4,987,743 and 6,694,749 B2. 
   This type of valve has the disadvantage of producing wear dust from the valve disc and/or valve seat. The wear dust from the valve disc tends to be blown into the cold head itself, which degrades performance. The pulse tube refrigerator is more sensitive to the dust than a conventional GM refrigerator because this dust tends to stick on the surface of the needles which are used to adjust the opening of the orifices at the warm end of the pulse tube, or to accumulate in the orifices and flow passages. The performance of a pulse tube refrigerator is sensitive to the opening of the orifices, thus it is desirable to keep them free of dust. 
   It has now been found that a rotary valve unit can be designed, that uses a thrust bearing to support the rotating valve disc relative to the valve seat such that the gap between them varies from light contact to a very small gap. This results in very little or no wear, and the torque required to turn the valve disc is reduced. 
   SUMMARY 
   This invention provides an improved means of reducing the wear dust and the torque required to turn a multi-port rotary disc valve by maintaining very light contact or a very small gap between the face of the valve disc and the seat. This invention provides means to reduce the wear dust and the torque by having a thrust bearing hold the valve seat and/or disc such that they are not in contact with each other, or have light contact each other. 
   The gap between the face of valve disc and seat can be maintained between 0 to 25 μm so that the leakage from high pressure to low pressure is small enough that it does not affect the performance of the refrigerator. If the valve disc is in light contact with the valve seat, most of the force will be exerted on the face of the thrust bearing instead of the face of the valve seat. Since the face of the valve disc and the face of thrust bearing rotate together, no wear will be generated during rotation and the torque required to turn the valve disc can be small. 
   The thrust bearing can be attached to the valve seat or the valve disc by a friction fit, or it can be attached with adhesive. The thrust bearing can also be held in position by a fixture. 
   It is possible to further reduce the torque required to turn a rotary disc valve that has multiple ports by reducing the net force that keeps the face of the valve disc in contact with the face of the thrust bearing. This invention also provides means to reduce the axial force exerted on the thrust bearing by having gas at two different pressures acting on two different surfaces in the valve assembly as shown in U.S. Pat. Nos. 4,987,743 and 6,694,749. 
   It is also possible to have high-pressure gas in the center of the valve seat and low-pressure gas on the outside of the valve disc as shown in U.S. Pat. No. 6,694,749. This provides an additional advantage, especially in a multi-ported pulse tube, further reducing the amount of dust, from the wear of the valve disc, which is blown into the pulse tube. Having the high pressure in the center of the valve disc face and low pressure on the outside results in most of the dust being blown directly to the low-pressure space and never entering the pulse tube. 
   A valve unit can also be designed such that, during early operation, the valve disc is in contact with the valve seat, but the valve disc or the valve seat is not in contact with the thrust bearing. After the valve unit has run for some time and has experienced some wear, the valve disc or seat slowly comes into contact with the face of the thrust bearing. The load exerted on the thrust bearing thus increases gradually, which results in the load exerted on the engaged faces of the valve seat and disc decreasing gradually. Eventually, the load exerted on the engaged faces of the valve seat and disc will become 0, and no further wear will be generated. In this case, there is almost no gap between the face of valve seat and disc, therefore, the leak rate from high pressure to low pressure can be maintained at a very small flow rate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross section of a valve assembly in accordance with the present invention in which small schematics of the compressor and a single stage double inlet pulse tube refrigerator are included to show the flow relations. The valve seat is assembled with a thrust bearing attached; the valve disc is in contact with the surface of the thrust bearing. 
       FIG. 2  is a face view of a valve disc forming part of the valve unit of  FIG. 1 . 
       FIG. 3  is a face view of the valve seat forming part of the valve unit of  FIG. 1 . 
       FIG. 4  is a cross section of a second embodiment of a valve assembly in accordance with the present invention in which the thrust bearing is attached to the valve disc and the valve seat is in contact with the surface of the thrust bearing. 
       FIG. 5  is a cross section of a third embodiment of a valve assembly in accordance with the present invention in which the valve seat is assembled with a thrust bearing attached and the valve disc is in contact with the surface of the thrust bearing. Low pressure gas is introduced into a cavity enclosed by the valve disc and a valve holder. 
       FIG. 6  is a cross section of a fourth embodiment of a valve assembly in accordance with the present invention in which the valve disc is assembled with a thrust bearing attached and the valve seat is in contact with the surface of the thrust bearing. Low pressure gas is introduced into a cavity enclosed by the valve disc and a valve holder. 
       FIG. 7  is a cross section of a fifth embodiment of a valve assembly in accordance with the present invention in which the valve seat is assembled with a thrust bearing attached and the valve disc is in contact with the surface of the thrust bearing. High pressure gas is introduced into a cavity enclosed by the valve disc and a valve holder. 
       FIG. 8  is a cross section of a sixth embodiment of a valve assembly in accordance with the present invention in which the valve disc is assembled with a thrust bearing attached and the valve seat is in contact with the surface of the thrust bearing. High pressure gas is introduced into a cavity enclosed by the valve disc and a valve holder. 
       FIG. 9  is a cross section of a seventh embodiment of a valve assembly in accordance with the present invention in which the valve seat is assembled with a thrust bearing and the valve disc is in contact with the surface of the thrust bearing. The thrust bearing is fixed in the valve housing by a fixture. 
       FIG. 10  is a cross sections of the initial status of a design option for the first embodiment of a valve assembly in accordance with the present invention in which the valve seat is assembled with a thrust bearing attached, and the valve disc is in contact with the face of the valve seat, but not in contact with the face of the thrust bearing. 
       FIG. 11  is a cross sections of the valve assembly shown in  FIG. 10  after an initial wear period which brings the valve disc to the normal operating status in which the valve disc is in contact with the face of the thrust bearing, and most of the load is exerted on the face of the thrust bearing, not on the engaged face of valve seat. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is applicable to any kind of refrigerator in which gas is cycled in and out of the expander by a valve unit, including G-M refrigerators, Solvay refrigerators, and G-M type pulse tube refrigerators. It is of particular value when applied to low temperature pulse tubes that have multi-stages and multi-ports. 
     FIG. 1  shows a cross section of valve assembly  29  along with small schematics of the compressor and a single stage double inlet pulse tube refrigerator to show the flow relations. 
   Valve unit  29  has a valve motor assembly  5 , a valve housing  7  and a valve base  17 , all of which are sealed by means of a variety of ‘O’-ring seals, and by bolts  1 . Inside the valve base and housing, there are various components. A valve seat  21  is held and sealed within the valve housing. A thrust bearing  60  is assembled with the valve seat. A valve disc  4  is turned by valve motor  5  through a motor shaft  6  and a pin  3  passing through shaft  6 . Valve disc  4  is free to move axially relative to pin  3 . Valve disc  4  is in contact with the face of thrust bearing  60 . The valve disc  4  can be spaced apart from valve seat  21  by a very small gap or it can have very light contact with valve seat  21 . If there is a gap between the face of valve disc  4  and valve seat  21 , the preferred gap should be 0 to 25 μm. If the valve disc  4  is lightly in contact with valve seat  21 , most of the force should be exerted on the face of the thrust bearing  60  instead of the face of the valve seat  21 . Since the face of the valve disc  4  and the face of thrust bearing  60  rotate together, no wear will be generated during rotating and the required torque to drive the valve disc can be small. A spring  8  is used to keep valve disc  4  in contact with thrust bearing  60  when the refrigerator is off. Pin  35  prevents valve seat  21  from rotating relative to housing  17 . 
   An inlet  10  is connected to the supply side of compressor  20  through a gas line  19 . The return side of compressor  20  connects to valve assembly  29  through the gas line  18  and an outlet  14 . Gas at low pressure then flows out of the center of valve disc  4  through channel  13 . 
   The force, which is generated from the differential pressure between the supply pressure exerted on the distal face of the valve disc  4  and the pressure exerted on the face of valve disc  4 , keeps the face of the valve disc  4  in contact with the face of the thrust bearing  60 . 
     FIG. 2  shows the gas flow cavities in the face of valve disc  4 . The cross section shown in  FIG. 1  is noted by section arrows A-A in  FIGS. 2 and 3 . Gas from ports  15  flows into cavities  40  then to low-pressure, Pl, port  13  through cross slot  41 . Regions  12  that are under cut in the outer edge of valve disc  4  connect to high-pressure, Ph, gas that is supplied from the compressor 
     FIG. 3  shows the face of seat  21 . Although not essential to an understanding of the invention, the nature of this porting will be briefly described with reference to  FIGS. 1 ,  2 , and  3 .  FIG. 1  shows a double inlet type pulse tube refrigerator driven by the invented valve unit. It consists of a regenerator  22 , a pulse tube  25  with warm end flow smoother  26  and cold end flow smoother  24 , and a cold end heat exchanger  23 . A phase shifter, which includes a buffer volume  28 , a buffer orifice  27 , and a double inlet valve  30 . By rotating valve disc  4 , by means of valve motor  5  and shaft  6 , holes  15  and  16  are alternately pressurized by gas flowing through cavities  12  and depressurized by flow through slots  40 . The porting shown in  FIGS. 2 and 3  produce two complete cycles to pressurize and depressurize the pulse tube for every rotation of valve disc  4 . It is to be understood that the expander can be operated with one, or more than one, cycle per cycle of the rotary valve by properly arranging the supply and return porting on valve disc  4  and valve seat  21 . 
   Although the expander shown in  FIG. 1  is a single stage pulse tube, it is also possible to design the valve unit and porting so that it can be used to drive a multi-stage pulse tube with multiple control ports as shown for example in FIG. 9 of U.S. Pat. No. 6,256,998. By properly arranging the porting on the valve disc  4  and the valve seat  21 , and by arranging necessary passages to communicate with the warm end  26  of the pulse tube  25 , the invented valve unit can also be used to drive any type of pulse tube refrigerator, such as, orifice type, four valve type, active-buffer type and five-valve type. It must be pointed out that this valve unit can be used for other kinds of refrigerators, such as GM or Solvay types. 
     FIG. 4  shows a second embodiment of the present invention in which thrust bearing  60  is attached to valve disc  4  and valve seat  21  is in contact with the surface of the thrust bearing  60 . In  FIG. 4 , like references denote like parts in  FIG. 1 . 
     FIG. 5  shows a third embodiment of the present invention in which thrust bearing  60  is attached to valve seat  21  and valve disc  61  is in contact with the surface of thrust bearing  60 . In  FIG. 5 , like references denote like parts in  FIG. 1 . In  FIG. 5 , the force exerted on the thrust bearing  60  is reduced by having a valve holder  2  which is held by pin  3  and sealed in valve disc  61  by an ‘O’-ring  9 . The exterior surfaces of valve disc  61  and valve holder  2  are surrounded by high-pressure gas except for the surface of valve disc  61  that is in contact with thrust bearing  60  and the surface facing valve seat  21 . The force required to keep the face of valve disc  61  in contact with the face of thrust bearing  60  is obtained by having the product of the pressures and areas on the distal side of valve disc  61  be greater than the product of the maximum average pressure on the face of valve disc  61  and the area of the face of valve disc  61 . This can be expressed in the form of an equation in which Ac is the area of the distal side of valve disc  61  in cavity  11 , As is the annular area of the distal side of valve disc  61  around Ac, Av is the area of the face of valve disc  61 , and Pv is the average pressure acting on Av (both including the area and pressure of cavity  12 ), as
 ( Ac*Pl+As*Ph )&gt; Av*Pv max  Equation 1 
   The opposing force is transmitted to motor shaft  6  and puts an axial load on the motor bearings in the direction toward valve disc  61 . In practice the diameter of cavity  11  is adjusted by testing different sizes to see what gives the best balance between maintaining a seal and minimizing the load on thrust bearing  60 . 
     FIG. 6  shows a fourth embodiment of the present invention in which thrust bearing  60  is attached to valve disc  61  and valve seat  21  is in contact with the surface of the thrust bearing  60 . In  FIG. 6 , like references denote like parts in  FIG. 5 . 
     FIG. 7  shows a fifth embodiment of the present invention in which thrust bearing  60  is attached to valve seat  21  and valve disc  61  is in contact with the surface of the thrust bearing  60 . In  FIG. 7 , like references denote like parts in  FIG. 5 . In  FIG. 7 , the exterior surfaces of valve disc  61  and valve holder  2  are surrounded by low-pressure gas except for the surface of valve disc  61  that is in contact with thrust bearing  60  and the surface facing valve seat  21 . The force can be expressed in the form of an equation in which Ac is the area of the distal side of valve disc  61  in cavity  11 , As is the annular area of the distal side of valve disc  61  around Ac, Av is the area of the face of valve disc  61 , and Pv is the average pressure acting on Av (both including the area and pressure of cavity  12 ), as
 ( Ac*Ph+As*Pl )&gt; Av*Pv max  Equation 2 
   The opposing force is transmitted to motor shaft  6  and puts an axial load on the motor bearings in the direction away from valve disc  61 . In equations 1 and 2 Av is equal the sum of Ac and As. 
   Having high pressure in the center of the valve disc face and low pressure on the outside results in most of the dust being blown directly to the low-pressure space and never entering the pulse tube. 
     FIG. 8  shows a sixth embodiment of the present invention in which thrust bearing  60  is attached to valve disc  61  and valve seat  21  is in contact with the surface of the thrust bearing  60 . In  FIG. 8 , like references denote like parts in  FIG. 7 . 
     FIG. 9  shows a seventh embodiment of the present invention in which a fixture is used to fix thrust bearing  60  to valve housing  17 . Thrust bearing  60  rests on a shoulder of valve seat and valve disc  61  is in contact with the surface of thrust bearing  60 . In  FIG. 9 , like references denote like parts in  FIG. 1 . This embodiment has the advantage of easy replacement of the thrust bearing if maintenance is needed. 
     FIG. 10  and  FIG. 11  show an option of a means to fabricate the first embodiment of the present invention. During initial operation valve disc  4  is in contact with the face of valve seat  21 , but not in contact with the face of the thrust bearing  60 . After the valve unit has run for some time and has experienced some wear, valve disc  21  starts to be in contact with the face of the thrust bearing  60 . Then the load exerted on the thrust bearing  60  starts to increase gradually, which results in the load exerted on the engaged faces of valve seat  21  and disc  4  decreasing. At some point, the load exerted on the engaged faces of the valve seat  21  and disc  4  becomes 0 and no further wear will be generated. In this case, there is almost no gap between the face of valve seat  21  and disc  4 , therefore, the leak rate from high pressure to low pressure can be maintained at a very small value.