The present invention relates to a fluid flow device having asymmetric flow properties, and more particularly, to an asymmetric bidirectional fluid flow device for use in gas generation devices.
Molecular sieve pressure swing adsorption (PSA) oxygen concentrators are currently in use in home healthcare, industrial and aircraft oxygen applications for the purpose of generating oxygen. An exemplary PSA oxygen concentrator 8 is shown in FIG. 1a and includes an input/output (I/O) valve 10, connected to a bed 12, and depending on the position of valve 10, alternately receives air to be concentrated from a source (not shown), e.g., a compressor, and vents gases from the bed. Bed 12 contains a molecular sieve 30, i.e., an adsorptive material, for adsorbing non-oxygen constituents of the received air, e.g., water and nitrogen, and the bed is, in turn, connected to a check valve 14 to provide the generated product gas, e.g., oxygen and argon, for a particular application.
In operation, the feed air, i.e., the gas entering I/O valve 10 from a source, is supplied to bed 12 wherein unwanted gas constituents are captured in the adsorptive material, e.g., zeolite. The remaining gas exits bed 12 at port 15 and flows past check valve 14 as the product gas. Cyclically, the adsorptive material in bed 12 is regenerated, i.e., emptied of the captured unwanted gas constituents, by reversing the flow of air through bed 12. Thus, I/O valve 10 is manipulated so that entering gas from the source is blocked and gas is vented from bed 12 emptying the unwanted gas constituents from the adsorptive material. Typically, a portion of the product gas is used to backflush bed 12, i.e., reverse the flow of gas to remove the unwanted constituents, by providing the gas into port 15 and through bed 12 to vent through I/O valve 10. Upon emptying the unwanted gas constituents, the I/O valve 10 is reset to receive feed air and prevent approximately 30-50 psi, than the vent gas, e.g., approximately 1 psi, the fluid flow is known as asymmetric to persons in the art.
To improve the overall operation of PSA systems, most PSA systems use two or more beds of adsorptive material which are pressurized in a cyclic regenerative process. Thus, FIG. 1a includes a second I/O valve 16 connected to a second bed 18 having a port 19, in turn, connected to a second check valve 20. Additionally, a purge gas connection 22 connects the output connection 17 between first bed 12 and first check valve 14 to the output connection 21 between second bed 18 and second check valve 20.
As shown in FIG. 1a during a typical oxygen generating PSA cycle, pressurized gas flows through the first bed 12 of molecular sieve, i.e., adsorptive material, via I/O valve 10 while the second bed 18 is vented to atmosphere through I/O valve 16 (indicated by a dashed line). The pressurized first bed 12 adsorptive material preferentially adsorbs unwanted constituents such as water and nitrogen, allowing oxygen and argon to pass through to check valve 14. A portion of the oxygen-enriched gas passes through a check valve 14, where it is used as product gas, and the rest passes through purge gas connection 22 to back flush nitrogen and water from second bed 18 to atmosphere.
Then as shown in FIG. 1b, before first bed 12 becomes completely saturated with unwanted constituents, the first and second I/O valves 10, 16 are switched to supply feed air to second bed 18 and vent first bed 12 to atmosphere (shown as a dashed line). Second bed 18 then becomes the oxygen producing bed and first bed 12 is regenerated by venting to atmosphere via I/O valve 16.
Venting first bed 12 to atmosphere and back flushing with oxygen enriched gas completes the regeneration of the first bed. This cyclic regeneration process repeats continuously to enable the production of a controlled amount of gas, e.g. oxygen. Because the concentrator 8 uses the same flow line connecting bed 12 to I/O valve 10 for both inflow of feed air and outflow of exhaust gas, if the flow line for venting beds 12, 18 to atmosphere through I/O valves 10, 16 respectively is restricted or narrowed in order to reduce the velocity of the feed air, longer cycle times are required to adequately purge the beds 12, 18 which reduces the amount of product gas which can be produced from a given source of supply or feed air and a fixed amount of molecular sieve or adsorptive material.
Typically, PSA systems operate with inlet air pressures in the range of 30 to 50 psig and outlet pressures of less than 50 psig. In many cases, the PSA systems are optimized to reduce air consumption thereby enabling the use of smaller, lighter compressors.
In some applications, oxygen generated by the PSA oxygen concentrator is supplied to other medical devices, such as ventilators and anesthesia machines. In these applications, the product gas pressure, i.e., the pressure of the gas produced by the PSA oxygen concentrator and provided to the other medical devices, needs to be above the typical 50 psig outlet pressure to insure proper operation of the other medical devices. One previous method of providing gas at the required pressure is post-compressing the PSA oxygen concentrator product gas to the required pressure using a compressor. This prior approach is expensive, requiring the acquisition, maintenance, and use of a compressor in addition to the air source, and introduces additional failure points or modes.
Another approach is to pressurize the zeolite, i.e., the adsorptive material in the beds 12, 18, with higher inlet pressures from I/O valves 10, 16 in order to achieve the desired outlet pressure of the product gas. One of the disadvantages of this approach in the past has been the destruction of the zeolite during the pressure swing adsorption process at high pressures. This is particularly true at the inlet end 12A, 18A of the bed 12, 18 where the high velocity gas impinges on the molecular sieve 30. The impingement force is enough to cause failure of the zeolite or filtering media which is part of the molecular sieve 30 retention and a grinding action which grinds the sieve granules into a powder. The powder disadvantageously contaminates connecting lines and other components. Eventual dusting of the granules leads to bed failure.
Some manufacturers have sintered the zeolite in plastic pellets to prevent dusting, but this approach is expensive and requires additional weight and volume. Another approach has been to restrict the inlet and outlet flow of gas into the molecular sieve beds but this adversely affects the performance of the beds. In particular, restriction of the exhaust cycle, i.e., regeneration and venting of a bed, prevents complete regeneration of the beds and leads to performance degradation.
It is therefore an object of the present invention to provide a method and apparatus for enabling an asymmetric flow of fluid through a device.
Another object of the present invention is to enable an asymmetric, bidirectional flow of fluid through a device.
Another object of the present invention is to reduce the flow of fluid through a device in one direction while maximizing the flow of fluid through the device in another direction.
The above described objects are fulfilled by a method and apparatus for receiving a flow of fluid and restricting the flow of the fluid through a device. An apparatus aspect includes an insert having a shaped surface including at least one through-hole located off center. The shaped surface may be a bell-shaped curve in cross-section or a parabaloid, semi-spheroid, or elliptoid.
A method aspect includes supplying a flow of fluid to one side of an apparatus having a shaped surface and at least one through-hole located off-center, redirecting a portion of the flow of the fluid onto itself, and directing the flow to at least one through-hole.