Abstract:
The present invention reduces the amount of oxygen in an oxygen-containing gas within a closed environment. A selected amount of hydrogen gas is mixed with a portion of the oxygen-containing gas from the closed environment to form a first gas mixture. A catalyst exposed to the first gas mixture causes a reaction between the hydrogen and at least a portion of the oxygen therein. The resulting second gas mixture, which is returned to the closed environment, has a lower percentage of oxygen. At least one oxygen sensor is positioned in the closed environment to determine when oxygen levels in the closed environment reach a threshold level. The output signal from the sensor is used to control when and/or how much hydrogen is mixed in the first gas mixture.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of official duties by employees of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon. 
    
    
     CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This patent application is co-pending with one related patent application entitled “METHOD AND SYSTEM FOR HEATING AND HUMIDIFYING A BREATHABLE GAS”, Ser. No. 09/448,405, filed Nov. 22, 1999, and owned by the same assignee as this patent application. 
     FIELD OF THE INVENTION 
     The invention relates generally to the reduction or removal of a gas from a closed environment, and more particularly to a method and system for reducing oxygen levels in closed environments where oxygen toxicity or flammability is a concern. 
     BACKGROUND OF THE INVENTION 
     The U.S. Navy is committed to maintaining and improving its systems and techniques used to rescue survivors from a disabled submarine. Rescue operations can be required in a variety of situations to include those where the disabled submarine becomes internally pressurized due to flooding, leakage of compressed gas supplies, or through use of auxiliary breathing systems. 
     Efficient submarine rescue requires that pressurized crew members be decompressed more rapidly than current decompression procedures allow when using air. It has been shown that crew decompression can be accelerated significantly by having crew members pre-breath oxygen using, for example, face masks supplied with either pure oxygen or an oxygen-rich breathing gas. Unfortunately, such oxygen pre-breathing can result in oxygen buildup in the cabin atmospheres of the submarine through leakage around the face seal of the oxygen masks or leakage from the oxygen supply. High levels of oxygen can create hazardous conditions within the cabin atmosphere due to increased fire potential and/or oxygen toxicity. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method and system for reducing unsafe levels of oxygen in a closed environment. 
     Another object of the present invention to provide a method and system for monitoring and maintaining safe oxygen levels in a closed environment. 
     Still another object of the present invention to provide a method and system for monitoring and maintaining safe oxygen levels in a closed environment that is submerged in high ambient pressures. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, a method and system are provided for reducing the amount of oxygen in an oxygen-containing gas within a closed environment. A controllable mixer, in response to a control signal, mixes a selected amount of hydrogen gas with a portion of the oxygen-containing gas from the closed environment to form a first gas mixture that includes hydrogen and oxygen. A catalyst coupled to the mixer receives the first gas mixture and causes a reaction between the hydrogen therein and at least a portion of the oxygen therein. As a result, a second gas mixture is formed. The second gas, which is then returned to the closed environment, has a lower percentage of oxygen than the first gas mixture. At least one oxygen sensor, positioned in the closed environment, is coupled to the mixer to generate the mixer&#39;s control signal when oxygen levels in the closed environment reach a threshold level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top-level block diagram of a system used to monitor and reduce oxygen levels in a closed environment in accordance with the present invention; 
         FIG. 2  is a schematic view of an embodiment of the mixer used in the present invention; 
         FIG. 3  is a schematic view of another embodiment of the mixer used in the present invention; and 
         FIG. 4  is a schematic view of an embodiment of a system used to monitor and reduce oxygen levels in a sealed underwater cabin in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to  FIG. 1 , a system for reducing oxygen levels in a closed environment is illustrated in functional block diagram form. More specifically, the closed environment is designated by box  100  and is representative of any closed cabin maintained in space, on the earth&#39;s surface or underwater. As will become apparent from the following description, the present invention is particularly useful for closed environments that cannot be vented directly to the surrounding environment. Accordingly, by way of illustrative example, the features of the present invention will be described generally with respect to closed environments that are submerged in water. 
     In general, the present invention includes: a source  10  of hydrogen gas; a mixer  12  coupled to source  10  and configured to receive therein (as referenced by arrow  14 ) an oxygen-containing gas maintained within closed environment  100  and a controlled amount of hydrogen gas from source  10 ; a reaction-facilitating catalyst  16  coupled to mixing chamber  12  for facilitating a reaction between hydrogen and oxygen present in mixer  12  to effectively reduce the oxygen level therein and return the reduced-oxygen gas  14 A to closed environment  100 ; and an oxygen sensor  18  coupled to mixer  12  to monitor oxygen levels in closed environment  100  and provide a control signal indicative thereof to mixer  12 . 
     Source  10  can be any suitable source of hydrogen such as a tank of pure pressurized or unpressurized hydrogen gas or a metal hydride material that can be treated to release hydrogen gas as is well known in the art. Source  10  can be maintained within (as shown) or outside of closed environment  100  depending on the particular application needs. For example, if closed environment  100  is an underwater cabin and source  10  is a container of pressurized hydrogen gas, it is preferred to keep source  10  out of closed environment  100  for safety reasons. 
     Mixer  12  is representative of any controllable device/system that mixes hydrogen gas from source  10  with oxygen-containing gas  14  from closed environment  100  whenever oxygen sensor  18  indicates that oxygen levels in closed environments  100  have achieved a threshold level, i.e., oxygen levels need to be reduced. Mixer  12  can be achieved in a variety of ways without departing from the scope of the present invention. For example, as illustrated in  FIG. 2 , mixer  12  could be realized by an open or flow-through chamber  120  having a fan  122  disposed at one end thereof for sucking in oxygen-containing gas  14  from closed environment  100 . A metering valve  124  is disposed between a (pressurized) hydrogen source  10  and chamber  120  to control the flow of hydrogen into chamber  120 . The opening/closing of metering valve  124  is controlled by a control signal received from oxygen sensor  18 . 
     Another embodiment of mixer  12  is illustrated in  FIG. 3  where flow-through chamber  120  has a controllable jet nozzle  126  disposed therein. A (pressurized) hydrogen source  10  is coupled to jet nozzle  126 . The opening/closing of jet nozzle  126  is controlled by a control signal received from oxygen sensor  18 . When jet nozzle  126  is opened so that pressurized hydrogen gas is injected into chamber  120 , oxygen-containing gas  14  is drawn into chamber  120  and mixed with the injected hydrogen gas. It is to be understood that other embodiments of mixer  12  can be used, and that those disclosed herein are not limitations of the present invention. 
     When oxygen sensor  18  detects a pre-determined threshold oxygen level in closed environment  100 , mixer  12  generates a gas mixture that consists of oxygen-containing gas  14  and hydrogen gas from source  10 . This gas mixture is provided or exposed to catalyst  16  that will facilitate a reaction between the hydrogen and oxygen constituents in the gas mixture. Specifically, catalyst  16  is selected to facilitate the reaction of each one mole of hydrogen with one-half mole of oxygen (1H 2 +½O 2 ). This reaction produces water vapor (H 2 O) and heat (103,968 Btu/mol H 2 ) within the gas mixture. Catalyst  16  can be any material that facilitates the above-described reaction. In tests, precious metal (e.g., palladium, platinum, etc.) catalysts have performed well. While the catalyst could make use of these precious metals in their pure form, cost considerations will generally dictate that the catalyst material be supported or suspended on some lesser expensive material such as carbon, alumina, ceramics, etc. For example, the catalyst material could be surface-deposited on granular or particle-sized particles of a support matrix such as 0.8% (weight percentage) palladium deposited on extruded pellets of carbon which is available commercially from Engelhard Corporation, Seneca, S.C. 
     From a safety perspective, the volume percentage of hydrogen must remain well below the level at which oxygen and hydrogen would combust as is well known in the art. To achieve such safety levels, the volume percentage of hydrogen gas mixed with oxygen-containing gas  14  is generally on the order of approximately 1% or less. 
     oxygen sensor  18  is representative of any suitable sensor and/or sensor system that can detect the desired threshold level of oxygen and generate a control signal for mixer  12  as described above. Although only one oxygen sensor  18  is shown, there will typically be multiple oxygen sensors distributed throughout closed environment  100 . Thus, the particular type and number of oxygen sensors are not limitations of the present invention. 
     By way of illustrative example, an embodiment of the present invention useful for monitoring and reducing oxygen levels in a sealed and submerged underwater cabin  200  is illustrated in  FIG. 4 . A pressurized source  30  of hydrogen gas is maintained outside of closed environment  200  for safety reasons. A metering valve  32  couples source  30  to a flow-through chamber  34  maintained in closed environment  200 . Metering valve  32  can be maintained outside of or inside closed environment  200 . Chamber  34  could be part of the ventilation system (e.g., duct work) used in closed environment  200 . Disposed in chamber  34  is a catalyst  36  analogous to catalyst  16  described above. An oxygen sensor  38  disposed in closed environment  200  has its control signal output coupled to metering valve  32 . The operation of sensor  38  and metering valve  32  is analogous to that described above with respect to sensor  18  and metering valve  124 . A pressure sensor  40 , disposed to measure ambient pressure inside closed environment  200 , is coupled to oxygen sensor  38  for reasons that will be explained further below. 
     In operation of the system illustrated in  FIG. 4 , when oxygen sensor  38  detects an unacceptable level of oxygen in closed environment  200 , metering valve  32  is opened and oxygen-containing gas  14  flowing through chamber  34  is mixed with injected hydrogen gas. This gas mixture is exposed to catalyst  36  where the ensuing hydrogen/oxygen reaction reduces the oxygen in the gas  14 A returned to closed environment  200 . 
     Since closed environment  200  is assumed to be underwater, ambient cabin pressure within closed environment  200  factors into what level of oxygen is unacceptable. For example, at surface pressure (1 atmosphere), acceptable oxygen levels comprise a 16–50% oxygen content in the breathing gas. However, note that in determining acceptable levels of oxygen, one must also take into consideration that fire hazards in the cabin increase directly with oxygen percentage in the atmosphere. Typically, a maximum of 25% oxygen is recommended in the cabin atmosphere to minimize fire hazards. 
     In contrast to surface pressure conditions, elevated atmospheric pressures that may exist in a flooded submarine cause the acceptable oxygen percentages to be reduced inversely to pressure. That is, if the cabin atmosphere is twice that of normal atmospheric pressure, then the acceptable oxygen content will be halved to 8–25% oxygen; if the cabin pressure is five times normal pressure, then the oxygen content will be one fifth; etc. Accordingly, measurements of ambient cabin pressure by pressure sensor  40  can be used to change the threshold level of oxygen sensor  38  so that the present invention adapts to changing ambient cabin pressure conditions. 
     The advantages of the present invention are numerous. Oxygen levels in a closed environment are monitored and maintained to provide a safe breathable atmosphere for the closed environment&#39;s occupants. The system and method are simple and can be adapted to any closed environment used in space, on the ground or underwater. 
     Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.