Submarines operate as sealed underwater vessels, presenting a unique challenge in maintaining a breathable atmosphere for their crews. Unlike surface ships, a submerged submarine must independently manage its internal air composition. This involves continuously supplying oxygen for respiration and effectively removing exhaled carbon dioxide and other airborne impurities. Controlling the internal atmosphere is fundamental to a submarine’s operational endurance.
Generating Oxygen Onboard
Submarines primarily generate oxygen through water electrolysis. This method involves passing an electrical current through desalinated water, separating water molecules (H₂O) into hydrogen (H₂) and oxygen (O₂) gases. Seawater is first desalinated, often by heating it to create water vapor and then condensing it. The oxygen produced is channeled into the submarine’s atmosphere, while the hydrogen byproduct is vented safely into the ocean.
Electrolysis offers a continuous oxygen supply, particularly for nuclear-powered submarines with abundant electricity. As a backup or for emergencies, submarines also carry chemical oxygen generators, commonly known as oxygen candles. These cylindrical devices contain a mixture of sodium chlorate (NaClO₃) and iron powder. When ignited, the iron powder burns, generating heat that triggers a thermal decomposition reaction, releasing oxygen gas into the air.
Removing Carbon Dioxide
Removing exhaled carbon dioxide (CO₂) is as crucial as oxygen generation to prevent its accumulation to toxic levels. Submarines use specialized CO₂ scrubbers for this purpose. One common type is the amine scrubber, which uses a solution of monoethanolamine (MEA) to absorb CO₂ from the air. This process is regenerative; the MEA solution absorbs CO₂ when cool and releases it when heated, allowing the CO₂ to be compressed and discharged overboard, and the MEA to be reused.
Another method for CO₂ removal, often for backup or emergencies, involves chemical absorbents like soda lime or lithium hydroxide (LiOH). Soda lime, a mixture of calcium hydroxide and sodium hydroxide, chemically reacts with CO₂ to trap it. These granular absorbents are in canisters; once saturated, they are discarded and replaced. Lithium hydroxide canisters similarly absorb CO₂ without requiring electricity, making them suitable for power outages.
Maintaining Air Purity
Beyond oxygen and carbon dioxide, a submarine’s enclosed environment requires managing other airborne contaminants. Human activities, cooking, and machinery introduce trace gases like carbon monoxide (CO), hydrogen, and volatile organic compounds (VOCs). Catalytic burners oxidize harmful gases such as CO and hydrogen, converting them into less harmful substances like carbon dioxide and water vapor. These burners operate at elevated temperatures, requiring significant electrical power.
Activated carbon filters also maintain air purity by adsorbing odors, hydrocarbons, and other large-molecule organic compounds. These filters improve air quality and prevent the accumulation of substances that could overtax other purification systems. Continuous air monitoring systems track gas levels, ensuring all atmospheric components remain within safe limits for the crew.
Enabling Long-Term Submersion
A submarine’s submerged duration directly links to its internal atmosphere management, which varies significantly based on its propulsion system. Nuclear-powered submarines have a substantial advantage due to their onboard nuclear reactors, which generate ample electricity. This continuous power supply allows them to operate electrolysis units constantly, producing oxygen and running CO₂ removal systems for extended periods. The primary limitation for nuclear submarines’ submerged endurance then becomes the amount of food and other consumables they can carry, often allowing them to stay underwater for months, sometimes up to 120 days or more.
In contrast, diesel-electric submarines rely on batteries for submerged operations, which must be recharged periodically. Their diesel engines require oxygen, meaning these submarines must periodically surface to extend a snorkel mast. This “snorkeling” allows them to draw in fresh air for diesel engine operation, battery recharging, and boat ventilation. Consequently, diesel-electric submarines have a much shorter submerged endurance, typically needing to surface or snorkel every few days, though some advanced designs with air-independent propulsion (AIP) can extend this to a few weeks.