Submarines operate in a unique and challenging environment, requiring sophisticated systems to sustain human life for extended periods beneath the ocean’s surface. Maintaining a breathable atmosphere is fundamental, as these vessels are completely sealed off from external air. This necessitates an independent and continuous supply of oxygen, along with efficient methods to remove harmful byproducts of human respiration.
The Unique Challenge of Submarine Air
Life inside a submarine presents a fundamental atmospheric challenge due to its sealed nature. Crew members continuously consume oxygen and exhale carbon dioxide. This closed system leads to oxygen depletion and a dangerous accumulation of carbon dioxide, which can become toxic at elevated concentrations. Without active management, air quality would quickly deteriorate, making the environment uninhabitable.
Electrolysis: The Primary Oxygen Source
The primary method for generating oxygen in modern submarines is through electrolysis. This technique involves splitting water molecules (H₂O) into hydrogen (H₂) and oxygen (O₂).
Before electrolysis, seawater is typically distilled to remove impurities, as direct electrolysis of saltwater can produce toxic chlorine gas. The purified water, often with potassium hydroxide to improve conductivity, is fed into an electrolyzer. An electrical current passes through the water, causing molecules to break apart, with oxygen forming at one electrode and hydrogen at the other. The generated oxygen is collected and distributed or stored in high-pressure tanks. Hydrogen, an explosive byproduct, is typically vented safely overboard into the ocean, often through a diffuser for rapid dispersal. This continuous process allows submarines, especially nuclear-powered ones with abundant electricity, to remain submerged for months.
Chemical Oxygen Generators: Emergency Supply
In addition to electrolysis, submarines carry chemical oxygen generators, often called “oxygen candles,” as a supplementary or emergency oxygen source.
These cylindrical devices contain a mixture, typically sodium chlorate (NaClO₃) and iron powder. When ignited, usually by a firing pin, an exothermic reaction occurs, causing the mixture to smolder at high temperatures, around 600°C (1,112°F). This thermal decomposition releases oxygen at a fixed rate, along with byproducts like sodium chloride and iron oxide. Oxygen candles provide a rapid and independent source of oxygen, valuable when the primary electrolysis system is unavailable or additional oxygen is needed. However, their use is limited by their duration and the heat they generate during operation.
Managing Air Quality: Carbon Dioxide Removal
While oxygen generation is necessary, removing carbon dioxide is equally important for maintaining breathable air. Submarines employ various “scrubber” systems to chemically absorb CO₂ from the atmosphere. One common method utilizes soda lime, a mixture of calcium hydroxide and sodium hydroxide, which reacts with and traps CO₂. Lithium hydroxide canisters also offer a non-electric method for CO₂ absorption, useful during power outages.
For continuous CO₂ removal, many nuclear submarines utilize systems based on monoethanolamine (MEA), a liquid absorbent. CO₂-laden air is passed through a cool MEA solution, which absorbs the carbon dioxide. The CO₂-rich MEA solution is then heated, releasing the absorbed CO₂ in gaseous form. This concentrated CO₂ is compressed and discharged overboard into the seawater, allowing the regenerated MEA to be reused. Some advanced systems use solid sorbents like zeolites to capture CO₂, which can be regenerated by heating or vacuum.
Ensuring Crew Safety: Monitoring and Regulation
Continuous monitoring of the submarine’s atmosphere is essential for crew safety. Sensors and analytical systems measure concentrations of oxygen, carbon dioxide, and other potential contaminants like hydrogen and carbon monoxide. These systems provide real-time data on air composition. This surveillance allows operators to adjust oxygen generation and CO₂ removal systems as needed, maintaining breathable air within specified safe limits, typically around 19% oxygen and below 3% carbon dioxide.