Abstract:
An apparatus for producing nitrox includes, in one embodiment of the invention, an oxygen enricher to produce oxygen-rich air and a mixing chamber coupled to the oxygen enricher. The mixing chamber is adapted to combine mixing air with the oxygen-rich air to produce nitrox. A method is disclosed that includes removing nitrogen from air to produce an oxygen-nitrogen mixture having an oxygen concentration greater than desired and diluting the oxygen-nitrogen mixture with mixing air having a less than desired oxygen concentration to obtain nitrox having a desired oxygen concentration.

Description:
[0001]     This application claims benefit to a provisional application No. 60/507,665 filed on Sep. 30, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to gas pressure systems, and more particularly to systems that produce nitrox.  
         [0004]     2. Description of the Related Art  
         [0005]     Oxygen-rich air (nitrox) and oxygen-nitrogen-third gas mixes (trimix gases) are used in many applications, including scientific, commercial and military systems. One popular application for such gases is diving. By reducing the nitrogen content of air utilized by divers, such as by using nitrox or trimix gas in their breathing apparatus, longer bottom times may be obtained without extending the decompression time necessary after such dives. With longer bottom times, nitrox or trimix gas can be used to increase the safety margin during dives to reduce the risk of decompression sickness.  
         [0006]     Unfortunately, the production of nitrox and trimix gas can be hazardous if produced by blending oxygen (O 2 ) with the other gases due to the highly flammable character of the O 2  component. Also, blending operators must often pass a training and certification process prior to employment for nitrox and trimix production that increases the costs of production.  
         [0007]     One solution to reduce the dangers associated with producing nitrox is described in U.S. Pat. Nos. 5,846,291, 5,858,064, and 5,611,845 by W. Delp, II. In each of the patents, a permeable membrane gas separation system separates a nitrogen gas component from compressed air to produce nitrox for later storage and use. Although the systems reduce the flammability problems associated with O 2  and N 2  blending, production repeatability can be a problem because similar input pressures can result in dissimilar oxygen concentrations of the nitrox.  
         [0008]     One product that addresses the problems associated with O2 and N2 blending is the DNAx Nitrox Membrane System manufactured by Undersea Breathing Systems, Inc. The system has a semi-permeable gas separation membrane with a nitrogen-discharge backpressure controlled by a needle valve to positively control the oxygen concentration of permeate leaving the membrane. Because the oxygen concentration depends on adjustment of the needle valve, system repeatability is made more difficult. Also, the system downstream of the membrane requires positive and constant pressure to accurately measure the oxygen concentration of the nitrox entering the compressor due to a positive pressure requirement of the oxygen analyzer. When the system is cold and positive pressure has not developed, a high oxygen concentration may develop without the knowledge of the system&#39;s operator that is incompatible with standard oil-based compressors.  
         [0009]     A need continues to exist for a system to produce nitrox and trimix gas that reduces training requirements and the risks associated with traditional O 2  and N 2  blending techniques, and that enhances repeatability for consumer use.  
       SUMMARY OF THE INVENTION  
       [0010]     A method for producing nitrox includes removing nitrogen from air to produce an oxygen-nitrogen mixture having an oxygen concentration greater than desired and diluting the oxygen-nitrogen mixture with mixing air having a less than desired oxygen concentration to obtain nitrox having a desired oxygen concentration.  
         [0011]     An apparatus is disclosed for producing nitrox that includes an oxygen enricher to produce oxygen-rich air and a first mixing chamber coupled to the oxygen enricher, the mixing chamber adapted to combine mixing air with the oxygen-rich air to produce nitrox. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Like reference numerals designate corresponding parts throughout the different views.  
         [0013]      FIG. 1  is a perspective view of one embodiment of the invention that has a mixing chamber adapted to receive oxygen-rich air from an oxygen enricher to produce nitrox.  
         [0014]      FIG. 2  is a perspective view of a system for producing and storing nitrox that uses the embodiment of the invention illustrated in  FIG. 1 .  
         [0015]      FIG. 3  is a perspective view of the embodiment of the invention illustrated in  FIG. 2 , with a first trimix adapter coupled downstream of the mixing chamber.  
         [0016]      FIG. 4  is a perspective view of the embodiment of the invention illustrated in  FIG. 2 , with a second trimix adapter coupled downstream of the mixing chamber.  
         [0017]      FIG. 5  is a flow diagram illustrating a method of producing nitrox in accordance with one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     An apparatus for producing nitrox, in accordance with an embodiment of the invention, has an oxygen enricher coupled to a mixing chamber. The oxygen enricher is, preferably, a canister containing a semi-permeable gas separation membrane (“membrane”) that produces oxygen-rich air at a rate dependent on its input pressure and at a predetermined oxygen concentration. The mixing chamber combines mixing air, preferably ambient air, with the oxygen-rich air to produce nitrox. Because the 1) rate of production of oxygen-rich air is controlled by controlling the input pressure, 2) oxygen-rich air is produced and mixed with ambient air at ambient to slightly negative gage pressure and 3) the oxygen concentration of the oxygen-rich air is reduced to 40% or less prior to compression for storage, nitrox is produced with oxygen concentrations that are repeatable verses input pressure, favorable for use with standard oil-based compressors and without blending oxygen with nitrogen, for a safe and efficient system and method of nitrox production.  
         [0019]      FIG. 1  illustrates one embodiment of the invention that has the membrane coupled between a temperature stabilizer and mixing chamber to produce nitrox from ambient air. The membrane  102  is coupled at its input port  104  to an air-input hose  106  and at its oxygen-output port  108  to a mixing chamber  110  through a membrane-output line  112 . The membrane-output line  112  can include a vacuum pump or blower  114  to draw oxygen-rich air at ambient to slightly negative gage pressure from the oxygen-output port  108 . A nitrogen-output port  116  from the membrane is either connected to a nitrogen-output orifice  117  that is fixed, or is itself implemented with a fixed orifice, to allow nitrogen-rich air to escape.  
         [0020]     More particularly, the membrane  102  has a plurality of hollow tube fibers that allow O 2  to permeate faster than N 2  through their walls. Oxygen-rich air is drawn from the oxygen-output port  108  after permeation through the hollow tubes, while nitrogen-rich air continues through the hollow tubes to exit the membrane  102  at a pressure slightly less than the input pressure through the nitrogen-output port  116 . Because of the risk of fire when standard-oil pumps or compressors are used with oxygen concentrations greater than 40%, orifice size is designed and fixed so that the oxygen concentration of the oxygen-rich air is 40-50%, preferably 44%. Examples of a semi-permeable gas separation membrane include any of the PRISM® membranes sold by Air Products and Chemicals, Inc.  
         [0021]     Mixing air having a lower-than-desired oxygen concentration, preferably ambient air, enters the mixing chamber  110  through an air intake filter  118  at one end of the chamber. The air intake filter  118  reduces particulate contamination prior to the ambient air&#39;s introduction to a mixing portion  120  of the chamber. The mixing portion  120  is preferably a static mixing tube, a tube that has active mixing parts, or a corrugated hose to mix the oxygen-rich air from the membrane  102  with air introduced through the air intake filter  118  to produce nitrox at a desired O 2  concentration. Preferably, the nitrox is a homogenous mix of the ambient and oxygen-rich air that does not exceed an oxygen-concentration of 40% at the membrane&#39;s maximum output rating. The nitrox is sampled by a nitrox-oxygen sensor  122  coupled inline and downstream from the mixing portion  120  to determine its oxygen content.  
         [0022]     A temperature stabilizer  124 , preferably a thermostatically controlled heater with a thermostat control  126 , or a radiator or heat exchanger, prevents temperature fluctuations and conditions the air introduced to the membrane  102 . Preferably, for an oxygen enricher that is the membrane  102  described above, the air introduced into the input port  104  from the temperature stabilizer  124  is pressurized to 0.4-2.1 mega-Pascal (MPA) with its temperature approximately constant and between 15-54 degrees Celsius. If cooler air is provided to the membrane  102 , a lower volume of oxygen-rich air is produced with a higher oxygen concentration. Similarly, if warmer air is input into the membrane  102 , a higher volume of oxygen-rich air is produced with a lower oxygen concentration.  
         [0023]      FIG. 2  shows one embodiment of  FIG. 1  in further detail. Either high or low-pressure air sources can be provided by high-pressure storage tanks  200  or a low-pressure volume tank  205 , respectively. If the high-pressure air sources  200  are used, a high-pressure regulator  210  regulates air stored at approximately 10.3-31.0 MPA down to approximately 0.4-2.1 MPA through a check valve  215  that restricts back flow into the regulator  210 . The regulated air is preferably routed through a high-pressure source valve  220  (rated to 2.1 MPA) to the temperature stabilizer  124  prior to its introduction into the membrane  102 .  
         [0024]     If a low-pressure source is used, the low-pressure tank  205  is fed by a low-pressure compressor  225  through a first cooler  230 . While the high-pressure air source preferably contains pre-filtered and grade “E” or better air, the low-pressure compressor  225  provides air that must be filtered from water and oil vapor prior the air&#39;s introduction to the membrane  102 . Because commercial air filters do not operate as efficiently at elevated temperatures, the first cooler  230  cools the air after heating caused by the low-pressure compressors  225  to enable more efficient coalescing and filtration. The first cooler  230  can be one of many different types of coolers, including a radiator style placed in front of the fan and pulley on the compressor, a radiator style with additional fans, a water/heat exchanger that uses fresh or sea water to cool the air in cooling tubes, or a refrigerated cooling type or swamp cooler heat exchanger that use the dry nitrogen gas expelled from the membrane  102  to create a cooling effect with moisture, without creating a back pressure on the nitrogen-output port  116 . The low-pressure volume tank  205  also enables moisture to separate from the air within the tank and accumulate at the bottom of the tank. Air from the low pressure tank  205  is introduced to a low-pressure regulator  235  through coalescing, fine polish and oil vapor removal filters  240 ,  245 , and  250 , respectively. The coalescing filter  250  removes moisture and particles larger than approximately 1.0 microns. The fine polish and oil vapor removal filters  245  and  250  remove particles greater than 0.01 microns and oil vapor to 0.003 PPM, respectively, to maintain the life and effectiveness of the membrane  102 . The filtered air preferably enters the temperature stabilizer  124  after passing through a second check valve  255  and low-pressure source valve  260 . High and low pressure source valves  220 ,  260  can be ball, gate or solenoid valves. Filtration provided by the coalescing, fine polish and oil vapor removal filters is preferably grade “D” quality or better.  
         [0025]     If the nitrox-oxygen sensor  122  indicates that the oxygen-concentration of the nitrox is lower than desired, the user can raise the input pressure until the nitrox-oxygen sensor  122  indicates the desired oxygen concentration. Similarly, lowering the input pressure would result in lowering the indicated oxygen concentration of the nitrox. Preferably, the thermostatically controlled heater  124  is set to a constant value for each use and the input pressure is used to adjust and predict the resulting oxygen concentration of the nitrox.  
         [0026]     Nitrox flows past the nitrox-oxygen sensor  122  at ambient to slightly negative gage pressure and is compressed by a nitrox compressor  265 . Because typical oil and moisture filters loose their effectiveness or are susceptible to damage at higher temperatures and moisture levels, the nitrox can be cooled by a second cooler  270  and introduced to a condensate separator  275  prior to introduction to breathing air/Nitrox grade filters  280  and  285 . Similar to the first cooler  230 , the second cooler  270  can be an air cooler, heat exchanger or refrigerated dryer. The nitrox is distributed to large or small volume nitrox-storage tanks  290  and  295  through a gas distribution panel  297 . A high-pressure bypass line  298  also allows the gas distribution panel  297  to receive high-pressure air directly from air/nitrox compressor  265  to recharge the high pressure tanks  200 .  
         [0027]      FIG. 3  illustrates the application of the invention to the production of trimix gas in addition to producing nitrox. In this embodiment, prior to introducing the nitrox to the nitrox compressor  265  as in  FIG. 2 , a blower or fan  300  draws the oxygen-rich air into a second mixing chamber  305  at a low pressure to be mixed with a third gas, preferably Helium (He), that is stored in a third-gas pressure tank  310 . The He is regulated down to approximately ambient pressure through a pressure regulator  315 , then proceeds through a flow meter  320  for visual indication of He flow to the second mixing chamber  305 .  
         [0028]     An oxygen sensor  325 , preferably coupled inline with the gas stream, is provided downstream of the second mixing chamber  305  to indicate the oxygen concentration of the oxygen-nitrogen-helium trimix gas. By comparing the oxygen contents of the trimix and nitrox gases, the concentration of He in the trimix gas can be calculated. For example, if the indicated oxygen concentration of the nitrox gas is 30%, the calculated nitrogen concentration of the nitrox gas would be approximately 70% (assuming that nitrogen makes up the remainder of gas in the input air to the membrane  102 ). If the oxygen sensor  325  indicates 15% oxygen in the trimix gas after introduction of helium (a reduction of 50% from that indicated before the introduction of He), the calculated nitrogen concentration would be 35% (70% reduced by 50%). Because the sum of the gas concentrations must equal 100%, the helium concentration of the trimix gas would be calculated at 50% (15% O 2 +35% N 2 +50% He 2 =100% total gas).  
         [0029]     A trimix hose  330  carries the trimix to the nitrox compressor  265  for compression and then to the second cooler  270 , condensate separator  275  and high-pressure filters  280  and  285  for cooling, condensation and filtering, respectively, prior to storage in large or small volume trimix-storage tanks  335  and  340 . The gas distribution panel can be used to distribute the trimix gas for storage.  
         [0030]      FIG. 4  illustrates the application of the invention to the production of trimix gas without the use of a second mixing chamber  305 . Similar to the embodiment illustrated in  FIGS. 1 and 2 , permeate from the membrane-output line  112  and mixing air through air intake filter  118  are introduced to the mixing chamber  110 . In the embodiment illustrated in  FIG. 4 , the third gas, preferably Helium (He), is supplied to the mixing chamber  110  through a third-gas-input hose  400  coupled to the mixing portion  120 . The HE is regulated down from the third-gas pressure tank  310  through the pressure regulator  315  that is coupled to the third-gas-input hose  400 . A needle valve  405  is positioned in line between the pressure regulator  315  and the mixing portion  120  to provide further flow control of the He through the third-gas-input hose  400 .  
         [0031]     During operation, the air/nitrox compressor  265  is turned on and the nitrox-oxygen sensor  122  calibrated to the oxygen concentration of ambient air, or 21%. For a desired He concentration of 50%, the needle valve  405  is opened slowly and adjusted until the indicated oxygen concentration at the nitrox-oxygen sensor  122  is one-half of 21%, or, 10.5%. Permeate from the membrane-output line  112  is introduced to the mixing portion  120  and, if the high-pressure storage tanks  200  are used, the regulator  210  can be adjusted to adjust the input pressure for the membrane  104  until the nitrox-oxygen sensor  122  indicates the desired oxygen concentration of nitrox, typically 18% or 21%. The user can raise the input pressure to increase the oxygen concentration indicated on the nitrox-oxygen sensor  122 . Similarly, lowering the input pressure would result in lowering the indicated oxygen concentration of the nitrox for storage.  
         [0032]      FIG. 5  is a flow diagram of a method of producing nitrox that can be practiced with the system of  FIG. 2 . The nitrox compressor  265  and temperature stabilizer  124  are turned on (block  500 ). If the high-pressure source  200  is used (block  505 ), the high-pressure source valve  220  is opened (block  510 ). If the low pressure source  205  is used (block  515 ), the low-pressure source valve  260  is opened (block  520 ). The applicable pressure regulator  210  or  235  is manually adjusted to set the input pressure within the utilized membrane&#39;s operating range, generally between 0.4-2.1 MPA (block  525 ), and the air is processed by the temperature stabilizer  124  (block  530 ) and introduced into the membrane  102  (block  535 ). Oxygen and nitrogen-rich air are discharged from respective ports  108  and  116  (blocks  540 ,  545 ) and ambient air is introduced through filter  118  and mixed with the oxygen-rich air to produce nitrox (block  550 ). The oxygen content of the nitrox is measured by oxygen sensor  122  and, if it is at the desired oxygen concentration (block  555 ), is distributed for use or storage (block  560 ). Otherwise, the user increases the input pressure using the associated pressure regulator, if a higher oxygen concentration is desired, or decreases the input pressure if a lower oxygen concentration is desired (block  525 ).  
         [0033]     While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.