Patent Publication Number: US-2010116362-A1

Title: Passive control valve

Description:
This invention relates generally to valves to control the release of gas from a pressurized vessel such as for use in a liquid droplet generation and delivery system, and more specifically concerns such a control valve which is passive in operation. 
     Pressurized vessels containing gas, such as air, are known for use in various applications. One example is for accelerating liquid droplets, such as a dentifrice or water droplets, used in a teeth cleaning system. In such an application, and similar applications involving the acceleration of liquid droplets, it is important that the mass flow of the gas from the pressurized vessel remain sufficiently constant over a selected period of time, typically during the entire time that the valve is open, from the vessel being full to the vessel being empty. 
     Heretofore, control valves to provide constant mass flow have been active rather than passive in operation. In an active valve, the exit area of the valve increases by means of a control circuit/assembly as a function of time, compensating for the reduced pressure of the gas as the vessel empties. It is possible to calculate mathematically the function which will accomplish this result. However, the control circuit/structure to carry out the function is complex; such a valve is expensive and will often be unreliable in use. 
     Hence, it would be desirable to have a simple, preferably passive, control valve to produce a constant mass flow rate from a pressurized vessel as it goes from full capacity to empty. 
    
    
     
       Accordingly, the present invention is a passive control valve for use with a pressurized gas vessel, comprising: a control valve for controlling the release of gas from a pressurized vessel, the released gas then being used to accelerate liquid droplets, wherein the control valve comprises a rubberized material having an opening therethrough, allowing gas to escape in a controlled manner from the pressurized vessel, wherein the rubberized material has such a Shore hardness value and wherein the valve has such a configuration, relative to the size of the opening therethrough the valve and the pressure of the gas in the vessel, that the mass flow of gas from the vessel is approximately constant over time as gas exits from the vessel. 
         FIG. 1  is a simple cross-sectional view showing the combination of a pressurized vessel, a control valve and a droplet generation system. 
         FIG. 2  is a cross-sectional view showing in more detail the embodiment of the control valve described herein. 
     
    
    
     In general, the present invention is a passive control valve  14  which in operation releases gas from a pressurized vessel with a mass flow rate which remains constant over a selected period of use, such as from the time that the release of gas begins until the gas is all released. The valve has an opening  26  which permits the release of gas. The valve itself is so constructed and arranged and comprises such a material that the area of the opening through the valve  14  is an inverse function of the reservoir pressure, i.e. as the pressure decreases, the cross-sectional area of the opening  26  increases. 
     Such a valve is useful in a variety of applications. One such application is a liquid droplet generation system, with the gas exiting from the control valve being used to accelerate liquid droplets to a desired high velocity. One example of the use of liquid droplets is for cleansing teeth, in which individual droplets of water or liquid dentifrice of a selected size are accelerated to a desired velocity and directed toward a user&#39;s teeth for cleansing thereof. Such a droplet system is shown and described in co pending application Ser. No. 60/537,690, filed on Jan. 20, 2004, which is owned by the assignee of the present invention, the contents of which are hereby incorporated by reference. It should be understood, however, that the present control valve can be used in other gas-assisted liquid droplet systems. This could include applications such as delivering medications transdermally by directing high speed droplets through the stratum corneum into the epidermis. 
     Still further, it should be understood that the control valve can be used in other applications where it is important to have a constant mass flow rate from a source of pressurized gas, such as air, or other gas, while the vessel is emptying. 
     A gas-assisted liquid droplet dentifrice system  10  is shown in  FIG. 1 . It includes a pressurized gas vessel  10 , which has been pressurized from an external source to a desired pressure, for example 50 Bar. Other pressures could, of course, be used. One such vessel is made from stainless steel, with a length of 0.14 m, an internal volume of 55 cc, an outer diameter of 3 cm and a wall thickness of 3 mm. The exit opening  12  from the pressurized vessel is controlled by a shut-off valve  13 . Downstream of shut-off valve  13  is a passive control valve  14 , located at the front end of a brass duct  16  which has a diameter in the embodiment shown of 5 mm, and a length of 25 cm. At the exit of duct portion  16  is represented a droplet generator  18 , such as shown and described in the above-referenced application. At this point, duct portion  16  narrows to duct portion  20  which has a diameter of approximately 0.5 mm. The droplets exit from duct portion  20  at a top member  21 , accelerated by the air stream from valve  14  and are directed toward a target, such as the teeth of a user. 
     During release of gas from the pressurized vessel, the difference in pressure between that inside the vessel and the atmospheric pressure is large enough to produce gas exiting at the speed of sound in the smallest portion of the exit field. The mass flow of air through the above system is determined by the following formula: 
     
       
         
           
             
               m 
               . 
             
             = 
             
               
                 
                   ρ 
                   o 
                 
                  
                 
                   KA 
                   c 
                 
               
               
                 
                   RT 
                   o 
                 
               
             
           
         
       
     
     Where K is a constant, ρ o  is the pressure in the vessel, T o  is the temperature in the pressurized vessel, 
         R=R   g   /M   m =287 J ·kg −1   ·K   −1  (for air) 
     with R g  being the universal gas constant, M m  the molar mass of air, and A c  is the area of the smallest part of the valve, where the air velocity is equal to the speed of sound. 
     During the release of gas, the pressure in the vessel  12  will decrease from its initial value (50 bar in the above example) to approximately atmospheric pressure. In view of the above formula, under isothermal conditions, a valve with a value of constant open area Ac will result in a decreasing gas flow as the pressure in the vessel decreases. This results in a decrease in gas velocity in the system. Such a condition is also true for adiabatic conditions. 
     Valve  14  in  FIG. 1  is shown in more detail in  FIG. 2 , and is a passive control valve which results in a constant mass flow over the time of release of gas from the vessel and corresponding decrease in pressure in vessel  10 . In general, valve  14  is conical in exterior configuration, with a small peripheral lip  24  at the base of the valve which contacts and seals against the interior surface  25  of duct portion  16  ( FIG. 1 ) leading from the pressurized tank. In the embodiment shown, for a vessel which is pressurized to 50 Bar, for example, a representative valve  14  is 11 mm long, with opening  26  through the valve having a diameter of 0.2 mm. The wall thickness of valve  14  increases from 6.7 mm at the end  28  proximal the pressurized vessel to approximately 10.6 mm at lip  24 . 
     In the embodiment shown, valve  14  is made from rubber, although it could be made from other flexible, resilient materials, such as, for instance, other elastomeric materials. For a pressurized vessel of 50 Bar, the hardness of the rubber is 50 Shore A. The configuration and structure of a rubberized valve results in a substantially constant mass flow volume from a pressurized vessel of 50 Bar. 
     In operation, air will flow through opening  26  in valve  14  at a substantially constant rate; at the end of opening  26 , the pressure will drop to a value close to atmospheric. Due to this pressure difference, rubber valve  14  will be initially deformed inwardly to some extent, resulting in a smaller local internal diameter. In the embodiment shown, the opening will be approximately 0.07 mm in diameter at the start. As the pressure in the vessel decreases, due to continuous exit of the gas, a reduction in the pressure difference occurs relative to atmospheric pressure This reduces the pressure on the control valve, letting it gradually open up to it&#39;s normal internal diameter. The rate of increase in the size of the opening in the valve shown for the pressurized vessel described is a sufficiently precise inverse function of the pressure remaining in the vessel, so that the mass flow of gas from the vessel remains constant during exit of the gas from the vessel. 
     The area A c  changes with the pressure in the vessel according to the formula: A c ∝P o   −α  For A c  to be a sufficiently precise inverse function of the pressure in the vessel, the value of inversion parameter α is preferably between 0.9 and 1.1, more preferably between 0.95 and 1.05, and most preferably between 0.98 and 1.02. 
     For a different pressurized vessel, with a different pressure, valve  14  could have a different length, shape and wall thickness, a different nominal opening diameter, and could comprise a different material, having a different hardness value. These factors are all adjusted to produce a constant mass flow over time for a particular pressurized vessel application. Again, for the particular vessel described above, the key consideration is the adjustment of the various physical characteristics of the control valve to correspond with the pressure so as to provide an inverse function of opening size v. pressure remaining in the vessel. 
     Another advantage of the rubberized passive control valve is that it dampens sound generated by the shock which results from the pressure decline from the value of pressure in the vessel to a pressure close to atmospheric conditions. 
     Accordingly, a passive pressure control valve has been shown and described which produces a constant mass flow for a pressurized vessel. 
     Although a preferred embodiment of the invention has been disclosed for purposes of illustration, it should be understood that various changes, modifications and substitutions may be incorporated in the embodiment without departing from the spirit of the invention which is defined by the claims which follow.