A submarine is a completely sealed environment, which presents a profound challenge to sustaining human life for extended periods. Unlike surface vessels that can simply open a window for fresh air, a submerged vessel must function as a self-contained ecosystem. The crew’s survival depends entirely on sophisticated life support systems that constantly replenish breathable air and remove toxic byproducts. This complex process involves creating a continuous cycle of air renewal, which is a significant feat of engineering designed to keep the atmosphere within safe and comfortable limits.
Generating the Breath of Life: Primary Oxygen Production
The primary method for continuously producing oxygen on modern, long-endurance submarines is a process called water electrolysis. This technique uses a steady electrical current to split water molecules (\(\text{H}_2\text{O}\)) into their constituent elements: hydrogen and oxygen. Since the surrounding seawater contains salt and other impurities that would produce toxic chlorine gas during electrolysis, the water must first be purified, often through distillation or reverse osmosis.
The purified water, sometimes mixed with an electrolyte like potassium hydroxide to increase conductivity, is then fed into an electrolytic oxygen generator. Within this machine, electricity breaks the chemical bonds of the water molecule, resulting in oxygen gas (\(\text{O}_2\)) and hydrogen gas (\(\text{H}_2\)). The oxygen is then carefully released into the submarine’s atmosphere to maintain the correct concentration for the crew to breathe.
The hydrogen byproduct is flammable and could be dangerous if allowed to accumulate, so it is safely managed and released. In the most common practice, the hydrogen gas is vented overboard into the surrounding ocean. This continuous, automated production system is highly reliable, especially on nuclear-powered submarines which have an abundant supply of electricity to power the generator indefinitely.
For emergency situations or when the primary electrolysis unit is offline, submarines carry a backup source of oxygen called oxygen candles. These are cylindrical chemical oxygen generators containing a mixture that includes sodium chlorate (\(\text{NaClO}_3\)) and iron powder. When ignited, the iron powder burns at a high temperature, around \(600^\circ\text{C}\) (\(1,112^\circ\text{F}\)), which is hot enough to trigger the thermal decomposition of the sodium chlorate.
This chemical reaction produces a steady flow of oxygen gas, along with sodium chloride (common salt) and iron oxide. The process is self-sustaining once initiated because the initial oxygen produced helps burn the iron powder, which in turn generates the heat needed to continue the decomposition of the chlorate. Oxygen candles are valued for their long shelf life and ability to produce a large volume of oxygen quickly without relying on the ship’s power or water supply.
Managing the Main Byproduct: Carbon Dioxide Scrubbing
While oxygen generation is necessary, removing the carbon dioxide (\(\text{CO}_2\)) exhaled by the crew is equally important for survival. If \(\text{CO}_2\) levels are allowed to build up, they can quickly cause incapacitation and death, making its removal a primary focus of air revitalization. The most common continuous method for this is the use of regenerative amine scrubbers, which utilize a chemical absorbent like monoethanolamine (MEA).
The \(\text{CO}_2\)-laden air is passed through the scrubber, where the MEA solution chemically binds with the carbon dioxide at a low temperature, typically around \(21^\circ\text{C}\) (\(70^\circ\text{F}\)). The clean air is then recirculated back into the submarine’s atmosphere. This process operates continuously to keep \(\text{CO}_2\) concentrations within a safe range.
The MEA solution, now saturated with \(\text{CO}_2\), is routed to a separate section where it is heated, usually to a temperature of around \(121^\circ\text{C}\) (\(250^\circ\text{F}\)). The increased heat reverses the chemical reaction, forcing the captured \(\text{CO}_2\) out of the solution as a gas. This regenerated MEA is then cooled and returned to the scrubber to absorb more \(\text{CO}_2\), making the system regenerative and capable of long-term operation.
The highly concentrated \(\text{CO}_2\) that has been released from the MEA must then be expelled from the submarine. This is typically done by pressurizing the gas and venting it into the surrounding ocean water, ensuring it does not re-enter the vessel’s atmosphere. This entire cycle allows the submarine to remain submerged for months without being limited by the need to discard saturated scrubbing material.
For situations where the main power is lost or the regenerative system fails, non-regenerative \(\text{CO}_2\) removal is used as a backup. This involves canisters containing a chemical compound such as lithium hydroxide (\(\text{LiOH}\)). When air is passed through these canisters, the \(\text{LiOH}\) chemically reacts with the \(\text{CO}_2\) to form lithium carbonate and water, effectively trapping the \(\text{CO}_2\) permanently.
Ensuring Long-Term Habitability: Trace Gas and Contaminant Control
Beyond oxygen and carbon dioxide management, a variety of other airborne contaminants accumulate in a sealed environment from machinery, cleaning products, cooking, and the crew itself. These trace gases, which include carbon monoxide, hydrogen sulfide, and volatile organic compounds (VOCs), must be actively removed to maintain a habitable atmosphere. If left unchecked, these substances can cause chronic health problems or acute toxicity.
The primary system for neutralizing many of these contaminants is the catalytic burner, sometimes referred to as a “burn-off oven.” This device draws air across a catalyst, often a material like Hopcalite, which is heated to a high temperature, typically between \(232^\circ\text{C}\) and \(315^\circ\text{C}\) (\(450^\circ\text{F}\) and \(600^\circ\text{F}\)). The heat and catalyst work together to oxidize toxic gases like carbon monoxide and hydrogen, converting them into products like \(\text{CO}_2\) and water vapor.
Working in conjunction with the burner are activated charcoal filters, which are large beds of adsorbent material. These filters are effective at capturing and removing larger-molecule organic compounds and VOCs that cause odors, which the catalytic burner may not fully process. The charcoal prevents these contaminants from building up in the air.
To further purify the air, especially from microscopic particles, electrostatic precipitators are employed. These devices use an electrical charge to attract and remove aerosols and particulates, such as smoke and dust. This multi-layered approach of continuous oxygen generation, \(\text{CO}_2\) removal, and contaminant scrubbing creates a safe and stable environment that allows submarines to operate underwater for months at a time.